HTML5

The rules in this section are complicated by the fact that although conforming documents will never contain nested form elements, it is quite possible (e.g. using a script that performs DOM manipulation) to generate documents that have such nested elements. They are also complicated by rules in the HTML parser that, for historical reasons, can result in a form-associated element being associated with a form element that is not its ancestor.

When a form-associated element is created, its form owner must be initialized to null (no owner).

When a form-associated element is to be associated with a form, its form owner must be set to that form.

When a form-associated element's ancestor chain changes, e.g. because it or one of its ancestors was inserted or removed from a Document, then the user agent must reset the form owner of that element. The HTML parser overrides this requirement when inserting form controls.

When a form-associated element's form attribute is set, changed, or removed, then the user agent must reset the form owner of that element.

When a form-associated element has a form attribute and the ID of any of the elements in the Document changes, then the user agent must reset the form owner of that form-associated element.

When a form-associated element has a form attribute and an element with an ID is inserted into or removed from the Document, then the user agent must reset the form owner of that form-associated element.

When the user agent is to reset the form owner of a form-associated element, it must run the following steps:

1. If the element's form owner is not null, and the element's form content attribute is not present, and the element's form owner is its nearest form element ancestor after the change to the ancestor chain, then do nothing, and abort these steps.

2. Let the element's form owner be null.

3. If the element has a form content attribute, then run these substeps:

   1. If the first element in the Document to have an ID that is case-sensitively equal to the element's form content attribute's value is a form element, then associate the form-associated element with that form element.

   2. Abort the "reset the form owner" steps.

4. Otherwise, if the form-associated element in question has an ancestor form element, then associate the form-associated element with the nearest such ancestor form element.

5. Otherwise, the element is left unassociated.

...
 <form id="a">
  <div id="b"></div>
 </form>
 <script>
  document.getElementById('b').innerHTML =
     '<table><tr><td><form id="c"><input id="d"></table>' +
     '<input id="e">';
 </script>
...

Form-associated elements have a form IDL attribute, which, on getting, must return the element's form owner, or null if there isn't one.

The name IDL attribute must reflect the name content attribute.

A form control that is disabled must prevent any click events that are queued on the user interaction task source from being dispatched on the element.

Constraint validation: If an element is disabled, it is barred from constraint validation.

The disabled IDL attribute must reflect the disabled content attribute.

4.10.19.3 A form control's value

Form controls have a value and a checkedness. (The latter is only used by input elements.) These are used to describe how the user interacts with the control.

To define the behaviour of constraint validation in the face of the input element's multiple attribute, input elements can also have separately defined values.

When an element with the autofocus attribute specified is inserted into a document, user agents should run the following steps:

1. Let target be the element's Document.

2. If target has no browsing context, abort these steps.

3. If target's browsing context has no top-level browsing context (e.g. it is a nested browsing context with no parent browsing context), abort these steps.

4. If target's browsing context had the sandboxed automatic features browsing context flag set when target was created, abort these steps.

5. If target's origin is not the same as the origin of the Document of the currently focused element in target's top-level browsing context, abort these steps.

6. If target's origin is not the same as the origin of the active document of target's top-level browsing context, abort these steps.

7. If the user agent has already reached the last step of this list of steps in response to an element being inserted into a Document whose top-level browsing context's active document is the same as target's top-level browsing context's active document, abort these steps.

8. If the user has indicated (for example, by starting to type in a form control) that he does not wish focus to be changed, then optionally abort these steps.

9. Queue a task that checks to see if the element is focusable, and if so, runs the focusing steps for that element. User agents may also change the scrolling position of the document, or perform some other action that brings the element to the user's attention. The task source for this task is the DOM manipulation task source.

Focusing the control does not imply that the user agent must focus the browser window if it has lost focus.

The autofocus IDL attribute must reflect the content attribute of the same name.

Constraint validation: If an element has a maximum allowed value length, its dirty value flag is true, its value was last changed by a user edit (as opposed to a change made by a script), and the code-point length of the element's value is greater than the element's maximum allowed value length, then the element is suffering from being too long.

User agents may prevent the user from causing the element's value to be set to a value whose code-point length is greater than the element's maximum allowed value length.

The action IDL attribute must reflect the content attribute of the same name, except that on getting, when the content attribute is missing or its value is the empty string, the document's address must be returned instead. The target IDL attribute must reflect the content attribute of the same name. The method and enctype IDL attributes must reflect the respective content attributes of the same name, limited to only known values. The encoding IDL attribute must reflect the enctype content attribute, limited to only known values. The noValidate IDL attribute must reflect the novalidate content attribute. The formAction IDL attribute must reflect the formaction content attribute, except that on getting, when the content attribute is missing or its value is the empty string, the document's address must be returned instead. The formEnctype IDL attribute must reflect the formenctype content attribute, limited to only known values. The formMethod IDL attribute must reflect the formmethod content attribute, limited to only known values. The formNoValidate IDL attribute must reflect the formnovalidate content attribute. The formTarget IDL attribute must reflect the formtarget content attribute.

When these methods and attributes are used with input elements while they don't apply, they must raise an INVALID_STATE_ERR exception. Otherwise, they must act as described below.

For input elements, these methods and attributes must operate on the element's value. For textarea elements, these methods and attributes must operate on the element's raw value.

Where possible, user interface features for changing the text selection in input and textarea elements must be implemented in terms of the DOM API described in this section, so that, e.g., all the same events fire.

The selections of input and textarea elements have a direction, which is either forward, backward, or none. This direction is set when the user manipulates the selection. The exact meaning of the selection direction depends on the platform.

On Windows, the direction indicates the position of the caret relative to the selection: a forward selection has the caret at the end of the selection and a backward selection has the caret at the start of the selection. Windows has no none direction. On Mac, the direction indicates which end of the selection is affected when the user adjusts the size of the selection using the arrow keys with the Shift modifier: the forward direction means the end of the selection is modified, and the backwards direction means the start of the selection is modified. The none direction is the default on Mac, it indicates that no particular direction has yet been selected. The user sets the direction implicitly when first adjusting the selection, based on which directional arrow key was used.

The select() method must cause the contents of the text field to be fully selected, with the selection direction being none, if the platform support selections with the direction none, or otherwise forward. The user agent must then queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

The selectionStart attribute must, on getting, return the offset (in logical order) to the character that immediately follows the start of the selection. If there is no selection, then it must return the offset (in logical order) to the character that immediately follows the text entry cursor.

On setting, it must act as if the setSelectionRange() method had been called, with the new value as the first argument; the current value of the selectionEnd attribute as the second argument, unless the current value of the selectionEnd is less than the new value, in which case the second argument must also be the new value; and the current value of the selectionDirection as the third argument.

The selectionEnd attribute must, on getting, return the offset (in logical order) to the character that immediately follows the end of the selection. If there is no selection, then it must return the offset (in logical order) to the character that immediately follows the text entry cursor.

On setting, it must act as if the setSelectionRange() method had been called, with the current value of the selectionStart attribute as the first argument, the new value as the second argument, and the current value of the selectionDirection as the third argument.

The selectionDirection attribute must, on getting, return the string corresponding to the current selection direction: if the direction is forward, "forward"; if the direction is backward, "backward"; and otherwise, "none".

On setting, it must act as if the setSelectionRange() method had been called, with the current value of the selectionStart attribute as the first argument, the current value of the selectionEnd attribute as the first argument, and the new value as the third argument.

The setSelectionRange(start, end, direction) method must set the selection of the text field to the sequence of characters starting with the character at the startth position (in logical order) and ending with the character at the (end-1)th position. Arguments greater than the length of the value in the text field must be treated as pointing at the end of the text field. If end is less than or equal to start then the start of the selection and the end of the selection must both be placed immediately before the character with offset end. In UAs where there is no concept of an empty selection, this must set the cursor to be just before the character with offset end. The direction of the selection must be set to backward if direction is a case-sensitive match for the string "backward", forward if direction is a case-sensitive match for the string "forward" or if the platform does not support selections with the direction none, and none otherwise (including if the argument is omitted). The user agent must then queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

All elements to which this API applies have either a selection or a text entry cursor position at all times (even for elements that are not being rendered). User agents should follow platform conventions to determine their initial state.

A listed form-associated element is a candidate for constraint validation except when a condition has barred the element from constraint validation. (For example, an element is barred from constraint validation if it is an output or fieldset element.)

An element can have a custom validity error message defined. Initially, an element must have its custom validity error message set to the empty string. When its value is not the empty string, the element is suffering from a custom error. It can be set using the setCustomValidity() method. The user agent should use the custom validity error message when alerting the user to the problem with the control.

An element can be constrained in various ways. The following is the list of validity states that a form control can be in, making the control invalid for the purposes of constraint validation. (The definitions below are non-normative; other parts of this specification define more precisely when each state applies or does not.)

Suffering from being missing

When a control has no value but has a required attribute (input required, select required, textarea required), or, in the case of an element in a radio button group, any of the other elements in the group has a required attribute.

Suffering from a type mismatch

When a control that allows arbitrary user input has a value that is not in the correct syntax (E-mail, URL).

Suffering from a pattern mismatch

When a control has a value that doesn't satisfy the pattern attribute.

Suffering from being too long

When a control has a value that is too long for the form control maxlength attribute (input maxlength, textarea maxlength).

Suffering from an underflow

When a control has a value that is too low for the min attribute.

Suffering from an overflow

When a control has a value that is too high for the max attribute.

Suffering from a step mismatch

When a control has a value that doesn't fit the rules given by the step attribute.

Suffering from a custom error

When a control's custom validity error message (as set by the element's setCustomValidity() method) is not the empty string.

An element can still suffer from these states even when the element is disabled; thus these states can be represented in the DOM even if validating the form during submission wouldn't indicate a problem to the user.

An element satisfies its constraints if it is not suffering from any of the above validity states.

4.10.21.2 Constraint validation

When the user agent is required to statically validate the constraints of form element form, it must run the following steps, which return either a positive result (all the controls in the form are valid) or a negative result (there are invalid controls) along with a (possibly empty) list of elements that are invalid and for which no script has claimed responsibility:

1. Let controls be a list of all the submittable elements whose form owner is form, in tree order.

2. Let invalid controls be an initially empty list of elements.

3. For each element field in controls, in tree order, run the following substeps:

   1. If field is not a candidate for constraint validation, then move on to the next element.

   2. Otherwise, if field satisfies its constraints, then move on to the next element.

   3. Otherwise, add field to invalid controls.

4. If invalid controls is empty, then return a positive result and abort these steps.

5. Let unhandled invalid controls be an initially empty list of elements.

6. For each element field in invalid controls, if any, in tree order, run the following substeps:

   1. Fire a simple event named invalid that is cancelable at field.

   2. If the event was not canceled, then add field to unhandled invalid controls.

7. Return a negative result with the list of elements in the unhandled invalid controls list.

If a user agent is to interactively validate the constraints of form element form, then the user agent must run the following steps:

1. Statically validate the constraints of form, and let unhandled invalid controls be the list of elements returned if the result was negative.

2. If the result was positive, then return that result and abort these steps.

3. Report the problems with the constraints of at least one of the elements given in unhandled invalid controls to the user. User agents may focus one of those elements in the process, by running the focusing steps for that element, and may change the scrolling position of the document, or perform some other action that brings the element to the user's attention. User agents may report more than one constraint violation. User agents may coalesce related constraint violation reports if appropriate (e.g. if multiple radio buttons in a group are marked as required, only one error need be reported). If one of the controls is not being rendered (e.g. it has the hidden attribute set) then user agents may report a script error.

4. Return a negative result.

The willValidate attribute must return true if an element is a candidate for constraint validation, and false otherwise (i.e. false if any conditions are barring it from constraint validation).

The setCustomValidity(message), when invoked, must set the custom validity error message to the value of the given message argument.

The validity attribute must return a ValidityState object that represents the validity states of the element. This object is live, and the same object must be returned each time the element's validity attribute is retrieved.

interface ValidityState {
  readonly attribute boolean valueMissing;
  readonly attribute boolean typeMismatch;
  readonly attribute boolean patternMismatch;
  readonly attribute boolean tooLong;
  readonly attribute boolean rangeUnderflow;
  readonly attribute boolean rangeOverflow;
  readonly attribute boolean stepMismatch;
  readonly attribute boolean customError;
  readonly attribute boolean valid;
};

A ValidityState object has the following attributes. On getting, they must return true if the corresponding condition given in the following list is true, and false otherwise.

valueMissing

The control is suffering from being missing.

typeMismatch

The control is suffering from a type mismatch.

patternMismatch

The control is suffering from a pattern mismatch.

tooLong

The control is suffering from being too long.

rangeUnderflow

The control is suffering from an underflow.

rangeOverflow

The control is suffering from an overflow.

stepMismatch

The control is suffering from a step mismatch.

customError

The control is suffering from a custom error.

valid

None of the other conditions are true.

When the checkValidity() method is invoked, if the element is a candidate for constraint validation and does not satisfy its constraints, the user agent must fire a simple event named invalid that is cancelable (but in this case has no default action) at the element and return false. Otherwise, it must only return true without doing anything else.

The validationMessage attribute must return the empty string if the element is not a candidate for constraint validation or if it is one but it satisfies its constraints; otherwise, it must return a suitably localized message that the user agent would show the user if this were the only form control with a validity constraint problem. If the user agent would not actually show a textual message in such a situation (e.g. it would show a graphical cue instead), then the attribute must return a suitably localized message that expresses (one or more of) the validity constraint(s) that the control does not satisfy. If the element is a candidate for constraint validation and is suffering from a custom error, then the custom validity error message should be present in the return value.

4.10.22.1 Introduction

4.10.22.2 Implicit submission

User agents may establish a button in each form as being the form's default button. This should be the first submit button in tree order whose form owner is that form element, but user agents may pick another button if another would be more appropriate for the platform. If the platform supports letting the user submit a form implicitly (for example, on some platforms hitting the "enter" key while a text field is focused implicitly submits the form), then doing so must cause the form's default button's activation behavior, if any, to be run.

Consequently, if the default button is disabled, the form is not submitted when such an implicit submission mechanism is used. (A button has no activation behavior when disabled.)

If the form has no submit button, then the implicit submission mechanism must just submit the form element from the form element itself.

4.10.22.3 Form submission algorithm

When a form element form is submitted from an element submitter (typically a button), optionally with a scripted-submit flag set, the user agent must run the following steps:

1. Let form document be the form's Document.

2. If form document has no associated browsing context or its browsing context had its sandboxed forms browsing context flag set when the Document was created, then abort these steps without doing anything.

3. Let form browsing context be the browsing context of form document.

4. If form is already being submitted (i.e. the form was submitted again while processing the events fired from the next two steps, probably from a script redundantly calling the submit() method on form), then abort these steps. This doesn't affect the earlier instance of this algorithm.

5. If the scripted-submit flag is not set, and the submitter element's no-validate state is false, then interactively validate the constraints of form and examine the result: if the result is negative (the constraint validation concluded that there were invalid fields and probably informed the user of this) then abort these steps.

6. If the scripted-submit flag is not set, then fire a simple event that is cancelable named submit, at form. If the event's default action is prevented (i.e. if the event is canceled) then abort these steps. Otherwise, continue (effectively the default action is to perform the submission).

7. Let form data set be the result of constructing the form data set for form.

8. Let action be the submitter element's action.

9. If action is the empty string, let action be the document's address of the form document.

   This step is a willful violation of RFC 3986, which would require base URL processing here. This violation is motivated by a desire for compatibility with legacy content. [RFC3986]

10. Resolve the URL action, relative to the submitter element. If this fails, abort these steps. Otherwise, let action be the resulting absolute URL.

11. Let scheme be the <scheme> of the resulting absolute URL.

12. Let enctype be the submitter element's enctype.

13. Let method be the submitter element's method.

14. Let target be the submitter element's target.

15. If the user indicated a specific browsing context to use when submitting the form, then let target browsing context be that browsing context. Otherwise, apply the rules for choosing a browsing context given a browsing context name using target as the name and form browsing context as the context in which the algorithm is executed, and let target browsing context be the resulting browsing context.

16. If target browsing context was created in the previous step, or if the form document has not yet completely loaded, then let replace be true. Otherwise, let it be false.

17. Select the appropriate row in the table below based on the value of scheme as given by the first cell of each row. Then, select the appropriate cell on that row based on the value of method as given in the first cell of each column. Then, jump to the steps named in that cell and defined below the table.

    	GET	POST
    http	Mutate action URL	Submit as entity body
    https	Mutate action URL	Submit as entity body
    ftp	Get action URL	Get action URL
    javascript	Get action URL	Get action URL
    data	Get action URL	Post to data:
    mailto	Mail with headers	Mail as body

    If scheme is not one of those listed in this table, then the behavior is not defined by this specification. User agents should, in the absence of another specification defining this, act in a manner analogous to that defined in this specification for similar schemes.

    The behaviors are as follows:

    Mutate action URL

    Let query be the result of encoding the form data set using the application/x-www-form-urlencoded encoding algorithm, interpreted as a US-ASCII string.

    Let destination be a new URL that is equal to the action except that its <query> component is replaced by query (adding a U+003F QUESTION MARK character (?) if appropriate).

    Navigate target browsing context to destination. If replace is true, then target browsing context must be navigated with replacement enabled.

    Submit as entity body

    Let entity body be the result of encoding the form data set using the appropriate form encoding algorithm.

    Let MIME type be determined as follows:

    If enctype is application/x-www-form-urlencoded

    Let MIME type be "application/x-www-form-urlencoded".

    If enctype is multipart/form-data

    Let MIME type be the concatenation of the string "multipart/form-data;", a U+0020 SPACE character, the string "boundary=", and the multipart/form-data boundary string generated by the multipart/form-data encoding algorithm.

    If enctype is text/plain

    Let MIME type be "text/plain".

    Otherwise, navigate target browsing context to action using the HTTP method given by method and with entity body as the entity body, of type MIME type. If replace is true, then target browsing context must be navigated with replacement enabled.

    Get action URL

    Navigate target browsing context to action. If replace is true, then target browsing context must be navigated with replacement enabled.

    Post to data:

    Let data be the result of encoding the form data set using the appropriate form encoding algorithm.

    If action contains the string "%%%%" (four U+0025 PERCENT SIGN characters), then %-escape all bytes in data that, if interpreted as US-ASCII, do not match the unreserved production in the URI Generic Syntax, and then, treating the result as a US-ASCII string, further %-escape all the U+0025 PERCENT SIGN characters in the resulting string and replace the first occurrence of "%%%%" in action with the resulting double-escaped string. [RFC3986]

    Otherwise, if action contains the string "%%" (two U+0025 PERCENT SIGN characters in a row, but not four), then %-escape all characters in data that, if interpreted as US-ASCII, do not match the unreserved production in the URI Generic Syntax, and then, treating the result as a US-ASCII string, replace the first occurrence of "%%" in action with the resulting escaped string. [RFC3986]

    Navigate target browsing context to the potentially modified action (which will be a data: URL). If replace is true, then target browsing context must be navigated with replacement enabled.

    Mail with headers

    Let headers be the resulting encoding the form data set using the application/x-www-form-urlencoded encoding algorithm, interpreted as a US-ASCII string.

    Replace occurrences of U+002B PLUS SIGN characters (+) in headers with the string "%20".

    Let destination consist of all the characters from the first character in action to the character immediately before the first U+003F QUESTION MARK character (?), if any, or the end of the string if there are none.

    Append a single U+003F QUESTION MARK character (?) to destination.

    Append headers to destination.

    Navigate target browsing context to destination. If replace is true, then target browsing context must be navigated with replacement enabled.

    Mail as body

    Let body be the resulting encoding the form data set using the appropriate form encoding algorithm and then %-escaping all the bytes in the resulting byte string that, when interpreted as US-ASCII, do not match the unreserved production in the URI Generic Syntax. [RFC3986]

    Let destination have the same value as action.

    If destination does not contain a U+003F QUESTION MARK character (?), append a single U+003F QUESTION MARK character (?) to destination. Otherwise, append a single U+0026 AMPERSAND character (&).

    Append the string "body=" to destination.

    Append body, interpreted as a US-ASCII string, to destination.

    Navigate target browsing context to destination. If replace is true, then target browsing context must be navigated with replacement enabled.

    The appropriate form encoding algorithm is determined as follows:

    If enctype is application/x-www-form-urlencoded

    Use the application/x-www-form-urlencoded encoding algorithm.

    If enctype is multipart/form-data

    Use the multipart/form-data encoding algorithm.

    If enctype is text/plain

    Use the text/plain encoding algorithm.

4.10.22.4 Constructing the form data set

The algorithm to construct the form data set for a form form is as follows:

1. Let controls be a list of all the submittable elements whose form owner is form, in tree order.

2. Let the form data set be a list of name-value-type tuples, initially empty.

3. Loop: For each element field in controls, in tree order, run the following substeps:

   1. If any of the following conditions are met, then skip these substeps for this element:

      • The field element has a datalist element ancestor.
      • The field element is disabled.
      • The field element is a button but it is not submitter.
      • The field element is an input element whose type attribute is in the Checkbox state and whose checkedness is false.
      • The field element is an input element whose type attribute is in the Radio Button state and whose checkedness is false.
      • The field element is not an input element whose type attribute is in the Image Button state, and either the field element does not have a name attribute specified, or its name attribute's value is the empty string.
      • The field element is an object element that is not using a plugin.

      Otherwise, process field as follows:

   2. Let type be the value of the type IDL attribute of field.

   3. If the field element is an input element whose type attribute is in the Image Button state, then run these further nested substeps:

      1. If the field element has a name attribute specified and its value is not the empty string, let name be that value followed by a single U+002E FULL STOP character (.). Otherwise, let name be the empty string.

      2. Let name_x be the string consisting of the concatenation of name and a single U+0078 LATIN SMALL LETTER X character (x).

      3. Let name_y be the string consisting of the concatenation of name and a single U+0079 LATIN SMALL LETTER Y character (y).

      4. The field element is submitter, and before this algorithm was invoked the user indicated a coordinate. Let x be the x-component of the coordinate selected by the user, and let y be the y-component of the coordinate selected by the user.

      5. Append an entry to the form data set with the name name_x, the value x, and the type type.

      6. Append an entry to the form data set with the name name_y and the value y, and the type type.

      7. Skip the remaining substeps for this element: if there are any more elements in controls, return to the top of the loop step, otherwise, jump to the end step below.

   4. Let name be the value of the field element's name attribute.

   5. If the field element is a select element, then for each option element in the select element whose selectedness is true, append an entry to the form data set with the name as the name, the value of the option element as the value, and type as the type.

   6. Otherwise, if the field element is an input element whose type attribute is in the Checkbox state or the Radio Button state, then run these further nested substeps:

      1. If the field element has a value attribute specified, then let value be the value of that attribute; otherwise, let value be the string "on".

      2. Append an entry to the form data set with name as the name, value as the value, and type as the type.

   7. Otherwise, if the field element is an input element whose type attribute is in the File Upload state, then for each file selected in the input element, append an entry to the form data set with the name as the name, the file (consisting of the name, the type, and the body) as the value, and type as the type. If there are no selected files, then append an entry to the form data set with the name as the name, the empty string as the value, and application/octet-stream as the type.

   8. Otherwise, if the field element is an object element: try to obtain a form submission value from the plugin, and if that is successful, append an entry to the form data set with name as the name, the returned form submission value as the value, and the string "object" as the type.

   9. Otherwise, append an entry to the form data set with name as the name, the value of the field element as the value, and type as the type.

   10. If the element has a form control dirname attribute, and that attribute's value is not the empty string, then run these substeps:

       1. Let dirname be the value of the element's dirname attribute.

       2. Let dir be the string "ltr" if the directionality of the element is 'ltr', and "rtl" otherwise (i.e. when the directionality of the element is 'rtl').

       3. Append an entry to the form data set with dirname as the name, dir as the value, and the string "direction" as the type.

       An element can only have a form control dirname attribute if it is a textarea element or an input element whose type attribute is in either the Text state or the Search state.

4. End: Return the form data step.

4.10.22.5 URL-encoded form data

The application/x-www-form-urlencoded encoding algorithm is as follows:

1. Let result be the empty string.

2. If the form element has an accept-charset attribute, then, taking into account the characters found in the form data set's names and values, and the character encodings supported by the user agent, select a character encoding from the list given in the form's accept-charset attribute that is an ASCII-compatible character encoding. If none of the encodings are supported, or if none are listed, then let the selected character encoding be UTF-8.

   Otherwise, if the document's character encoding is an ASCII-compatible character encoding, then that is the selected character encoding.

   Otherwise, let the selected character encoding be UTF-8.

3. Let charset be the preferred MIME name of the selected character encoding.

4. For each entry in the form data set, perform these substeps:

   1. If the entry's name is "_charset_" and its type is "hidden", replace its value with charset.

   2. If the entry's type is "file", replace its value with the file's filename only.

   3. For each character in the entry's name and value that cannot be expressed using the selected character encoding, replace the character by a string consisting of a U+0026 AMPERSAND character (&), a U+0023 NUMBER SIGN character (#), one or more characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) representing the Unicode code point of the character in base ten, and finally a U+003B SEMICOLON character (;).

   4. For each character in the entry's name and value, apply the appropriate subsubsteps from the following list:

      The character is a U+0020 SPACE character

      Replace the character with a single U+002B PLUS SIGN character (+).

      If the character isn't in the range U+0020, U+002A, U+002D, U+002E, U+0030 to U+0039, U+0041 to U+005A, U+005F, U+0061 to U+007A

      Replace the character with a string formed as follows:

      1. Let s be an empty string.

      2. For each byte b of the character when expressed in the selected character encoding in turn, run the appropriate subsubsubstep from the list below:

         If the byte is in the range 0x20, 0x2A, 0x2D, 0x2E, 0x30 to 0x39, 0x41 to 0x5A, 0x5F, 0x61 to 0x7A

         Append to s the Unicode character with the code point equal to the byte.

         Otherwise

         Append to the string a U+0025 PERCENT SIGN character (%) followed by two characters in the ranges U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) and U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F representing the hexadecimal value of the byte (zero-padded if necessary).

      Otherwise

      Leave the character as is.

   5. If the entry's name is "isindex", its type is "text", and this is the first entry in the form data set, then append the value to result and skip the rest of the substeps for this entry, moving on to the next entry, if any, or the next step in the overall algorithm otherwise.

   6. If this is not the first entry, append a single U+0026 AMPERSAND character (&) to result.

   7. Append the entry's name to result.

   8. Append a single U+003D EQUALS SIGN character (=) to result.

   9. Append the entry's value to result.

5. Encode result as US-ASCII and return the resulting byte stream.

4.10.22.6 Multipart form data

The multipart/form-data encoding algorithm is as follows:

1. Let result be the empty string.

2. If the algorithm was invoked with an explicit character encoding, let the selected character encoding be that encoding. (This algorithm is used by other specifications, which provide an explicit character encoding to avoid the dependency on the form element described in the next paragraph.)

   Otherwise, if the form element has an accept-charset attribute, then, taking into account the characters found in the form data set's names and values, and the character encodings supported by the user agent, select a character encoding from the list given in the form's accept-charset attribute that is an ASCII-compatible character encoding. If none of the encodings are supported, or if none are listed, then let the selected character encoding be UTF-8.

   Otherwise, if the document's character encoding is an ASCII-compatible character encoding, then that is the selected character encoding.

   Otherwise, let the selected character encoding be UTF-8.

3. Let charset be the preferred MIME name of the selected character encoding.

4. For each entry in the form data set, perform these substeps:

   1. If the entry's name is "_charset_" and its type is "hidden", replace its value with charset.

   2. For each character in the entry's name and value that cannot be expressed using the selected character encoding, replace the character by a string consisting of a U+0026 AMPERSAND character (&), a U+0023 NUMBER SIGN character (#), one or more characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) representing the Unicode code point of the character in base ten, and finally a U+003B SEMICOLON character (;).

5. Encode the (now mutated) form data set using the rules described by RFC 2388, Returning Values from Forms: multipart/form-data, and return the resulting byte stream. [RFC2388]

   Each entry in the form data set is a field, the name of the entry is the field name and the value of the entry is the field value.

   The order of parts must be the same as the order of fields in the form data set. Multiple entries with the same name must be treated as distinct fields.

   In particular, this means that multiple files submitted as part of a single <input type=file multiple> element will result in each file having its own field; the "sets of files" feature ("multipart/mixed") of RFC 2388 is not used.

   The parts of the generated multipart/form-data resource that correspond to non-file fields must not have a Content-Type header specified. Their names and values must be encoded using the character encoding selected above (field names in particular do not get converted to a 7-bit safe encoding as suggested in RFC 2388).

   The boundary used by the user agent in generating the return value of this algorithm is the multipart/form-data boundary string. (This value is used to generate the MIME type of the form submission payload generated by this algorithm.)

4.10.22.7 Plain text form data

The text/plain encoding algorithm is as follows:

1. Let result be the empty string.

2. If the form element has an accept-charset attribute, then, taking into account the characters found in the form data set's names and values, and the character encodings supported by the user agent, select a character encoding from the list given in the form's accept-charset attribute. If none of the encodings are supported, or if none are listed, then let the selected character encoding be UTF-8.

   Otherwise, the selected character encoding is the document's character encoding.

3. Let charset be the preferred MIME name of the selected character encoding.

4. If the entry's name is "_charset_" and its type is "hidden", replace its value with charset.

5. If the entry's type is "file", replace its value with the file's filename only.

6. For each entry in the form data set, perform these substeps:

   1. Append the entry's name to result.

   2. Append a single U+003D EQUALS SIGN character (=) to result.

   3. Append the entry's value to result.

   4. Append a U+000D CARRIAGE RETURN (CR) U+000A LINE FEED (LF) character pair to result.

7. Encode result using the selected character encoding and return the resulting byte stream.

4.10.23 Resetting a form

When a form element form is reset, the user agent must fire a simple event named reset, that is cancelable, at form, and then, if that event is not canceled, must invoke the reset algorithm of each resettable element whose form owner is form.

Each resettable element defines its own reset algorithm. Changes made to form controls as part of these algorithms do not count as changes caused by the user (and thus, e.g., do not cause input events to fire).