Home What are valid values for the id attribute in HTML?
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What are valid values for the id attribute in HTML?

Mr Shark
1#
Mr Shark Published in 2008-09-16 09:08:52Z

When creating the id attributes for HTML elements, what rules are there for the value?

Ivan Buttinoni
2#
Ivan Buttinoni Reply to 2016-04-02 17:09:56Z

For HTML 4, the answer is technically:

ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").

HTML 5 is even more permissive, saying only that an id must contain at least one character and may not contain any space characters.

The id attribute is case sensitive in XHTML.

As a purely practical matter, you may want to avoid certain characters. Periods, colons and '#' have special meaning in CSS selectors, so you will have to escape those characters using a backslash in CSS or a double backslash in a selector string passed to jQuery. Think about how often you will have to escape a character in your stylesheets or code before you go crazy with periods and colons in ids.

For example, the HTML declaration <div id="first.name"></div> is valid. You can select that element in CSS as #first\.name and in jQuery like so: $('#first\\.name'). But if you forget the backslash, $('#first.name'), you will have a perfectly valid selector looking for an element with id first and also having class name. This is a bug that is easy to overlook. You might be happier in the long run choosing the id first-name (a hyphen rather than a period), instead.

You can simplify your development tasks by strictly sticking to a naming convention. For example, if you limit yourself entirely to lower-case characters and always separate words with either hyphens or underscores (but not both, pick one and never use the other), then you have an easy-to-remember pattern. You will never wonder "was it firstName or FirstName?" because you will always know that you should type first_name. Prefer camel case? Then limit yourself to that, no hyphens or underscores, and always, consistently use either upper-case or lower-case for the first character, don't mix them.


A now very obscure problem was that at least one browser, Netscape 6, incorrectly treated id attribute values as case-sensitive. That meant that if you had typed id="firstName" in your HTML (lower-case 'f') and #FirstName { color: red } in your CSS (upper-case 'F'), that buggy browser would have failed to set the element's color to red. At the time of this edit, April 2015, I hope you aren't being asked to support Netscape 6. Consider this a historical footnote.

Peter Hilton
3#
Peter Hilton Reply to 2008-09-16 09:09:57Z

From the HTML 4 specification:

ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").

A common mistake is to use an ID that starts with a digit.

nasmorn
4#
nasmorn Reply to 2011-12-09 10:19:35Z

Strictly it should match

[A-Za-z][-A-Za-z0-9_:.]*

But jquery seems to have problems with colons so it might be better to avoid them.

Steve Morgan
5#
Steve Morgan Reply to 2008-09-16 09:12:19Z

From the HTML 4 spec...

ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").

EDIT: d'oh! Beaten to the button, again!

pdc
6#
pdc Reply to 2008-09-16 10:04:28Z

In practice many sites use id attributes starting with numbers, even though this is technically not valid HTML.

The HTML 5 draft specification loosens up the rules for the id and name attributes: they are now just opaque strings which cannot contain spaces.

pimvdb
7#
pimvdb Reply to 2011-05-31 06:33:40Z

Also, never forget that an ID is unique. Once used, the ID value may not appear again anywhere in the document.

You may have many ID's, but all must have a unique value.

On the other hand, there is the class-element. Just like ID, it can appear many times, but the value may be used over and over again.

Michael Thompson
8#
Michael Thompson Reply to 2013-12-30 15:03:14Z

You can technically use colons and periods in id/name attributes, but I would strongly suggest avoiding both.

In CSS (and several JavaScript libraries like jQuery), both the period and the colon have special meaning and you will run into problems if you're not careful. Periods are class selectors and colons are pseudo-selectors (eg., ":hover" for an element when the mouse is over it).

If you give an element the id "my.cool:thing", your CSS selector will look like this:

#my.cool:thing { ... /* some rules */ ... }

Which is really saying, "the element with an id of 'my', a class of 'cool' and the 'thing' pseudo-selector" in CSS-speak.

Stick to A-Z of any case, numbers, underscores and hyphens. And as said above, make sure your ids are unique.

That should be your first concern.

Álvaro González
9#
Álvaro González Reply to 2009-01-10 20:40:31Z

jQuery does handle any valid ID name. You just need to escape metacharacters (i.e., dots, semicolons, square brackets...). It's like saying that JavaScript has a problem with quotes only because you can't write

var name = 'O'Hara';

Selectors in jQuery API (see bottom note)

lstg
10#
lstg Reply to 2009-04-22 10:18:43Z

It appears that although colons (:) and periods (.) are valid in the HTML spec, they are invalid as id selectors in CSS so probably best avoided if you intend to use them for that purpose.

blacksun1
11#
blacksun1 Reply to 2010-07-07 10:09:22Z

Hyphens, underscores, periods, colons, numbers and letters are all valid for use with CSS and JQuery. The following should work but it must be unique throughout the page and also must start with a letter [A-Za-z].

Working with colons and periods needs a bit more work but you can do it as the following example shows.

<html>
<head>
<title>Cake</title>
<style type="text/css">
    #i\.Really\.Like\.Cake {
        color: green;
    }
    #i\:Really\:Like\:Cake {
        color: blue;
    }
</style>
</head>
<body>
    <div id="i.Really.Like.Cake">Cake</div>
    <div id="testResultPeriod"></div>

    <div id="i:Really:Like:Cake">Cake</div>
    <div id="testResultColon"></div>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
    <script type="text/javascript">
        $(function() {
            var testPeriod = $("#i\\.Really\\.Like\\.Cake");
            $("#testResultPeriod").html("found " + testPeriod.length + " result.");

            var testColon = $("#i\\:Really\\:Like\\:Cake");
            $("#testResultColon").html("found " + testColon.length + " result.");
        });
    </script>
</body>
</html>
Shashank N. Pandey
12#
Shashank N. Pandey Reply to 2012-09-22 18:27:51Z

alphabets-> caps & small
digits-> 0-9
special chars-> ':', '-', '_', '.'

the format should be either starting from '.' or an alphabet, followed by either of the special chars of more alphabets or numbers. the value of the id field must not end at an '_'.
Also, spaces are not allowed, if provided, they are treated as different values, which is not valid in case of the id attributes.

vsync
13#
vsync Reply to 2015-03-12 11:49:34Z

HTML5:

gets rid of the additional restrictions on the id attribute see here. The only requirements left (apart from being unique in the document) are:

  1. the value must contain at least one character (can’t be empty)
  2. it can’t contain any space characters.

PRE-HTML5:

ID should match:

[A-Za-z][-A-Za-z0-9_:.]*
  1. Must Start with A-Z or a-z characters
  2. May contain - (hyphen), _ (underscore), : (colon) and . (period)

but one should avoid : and . beacause:

For example, an ID could be labelled "a.b:c" and referenced in the style sheet as #a.b:c but as well as being the id for the element, it could mean id "a", class "b", pseudo-selector "c". Best to avoid the confusion and stay away from using . and : altogether.

Web Designer cum Promoter
14#
Web Designer cum Promoter Reply to 2013-05-02 06:51:33Z
  1. IDs are best suited for naming parts of your layout so should not give same name for ID and class
  2. ID allows alphanumeric and special characters
  3. but avoid using of # : . * ! symbols
  4. not allowed spaces
  5. not started with numbers or a hyphen followed by a digit
  6. case sensitive
  7. using ID selectors is faster than using class selectors
  8. use hyphen "-" (underscore "_" can also use but not good for seo) for long CSS class or Id rule names
  9. If a rule has an ID selector as its key selector, don’t add the tag name to the rule. Since IDs are unique, adding a tag name would slow down the matching process needlessly.
  10. In HTML5, the id attribute can be used on any HTML element and In HTML 4.01, the id attribute cannot be used with: <base>, <head>, <html>, <meta>, <param>, <script>, <style>, and <title>.
Kanishka Panamaldeniya
15#
Kanishka Panamaldeniya Reply to 2013-08-21 10:57:06Z

for HTML5

The value must be unique amongst all the IDs in the element’s home subtree and must contain at least one character. The value must not contain any space characters.

At least one character, no spaces.

This opens the door for valid use cases such as using accented characters. It also gives us plenty of more ammo to shoot ourselves in the foot with, since you can now use id values that will cause problems with both CSS and JavaScript unless you’re really careful.

Sergio
16#
Sergio Reply to 2013-10-09 06:31:00Z

HTML5

Keeping in mind that ID must be unique, ie. there must not be multiple elements in a document that have the same id value.

The rules about ID content in HTML5 are (apart from being unique):

This attribute's value must not contain white spaces. [...] 
Though this restriction has been lifted in HTML 5, 
an ID should start with a letter for compatibility.

This is the W3 spec about ID (från MDN):

Any string, with the following restrictions:
must be at least one character long
must not contain any space characters
Previous versions of HTML placed greater restrictions on the content of ID values 
(for example, they did not permit ID values to begin with a number).

More info:

  • W3 - global attributes (id)
  • MDN atribute (id)
Anthony
17#
Anthony Reply to 2013-11-13 20:14:15Z

To reference an id with a period in it you need to use a backslash. Not sure if its the same for hyphens or underscores. For example: HTML

<div id="maintenance.instrumentNumber">############0218</div>

CSS

#maintenance\.instrumentNumber{word-wrap:break-word;}
Sébastien
18#
Sébastien Reply to 2014-09-14 03:40:33Z

In HTML5, an id can't start with a number, e.g. id-"1kid" and they can't have spaces (id="Some kind")

Michael_B
19#
Michael_B Reply to 2016-12-05 18:29:53Z

HTML5: Permitted Values for ID & Class Attributes

As of HTML5, the only restrictions on the value of an ID are:

  1. must be unique in the document
  2. must not contain any space characters
  3. must contain at least one character

Similar rules apply to classes (except for the uniqueness, of course).

So the value can be all digits, just one digit, just punctuation characters, include special characters, whatever. Just no whitespace. This is very different from HTML4.

In HTML 4, ID values must begin with a letter, which can then be followed only by letters, digits, hyphens, underscores, colons and periods.

In HTML5 these are valid:

<div id="999"> ... </div>
<div id="#%LV-||"> ... </div>
<div id="____V"> ... </div>
<div id="⌘⌥"> ... </div>
<div id="♥"> ... </div>
<div id="{}"> ... </div>
<div id="©"> ... </div>
<div id="♤₩¤☆€~¥"> ... </div>

Just bear in mind that using numbers, punctuation or special characters in the value of an ID may cause trouble in other contexts (e.g., CSS, JavaScript, regex).

For example, the following ID is valid in HTML5:

<div id="9lions"> ... </div>

However, it is invalid in CSS:

From the CSS2.1 spec:

4.1.3 Characters and case

In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, two hyphens, or a hyphen followed by a digit.

In most cases you may be able to escape characters in contexts where they have restrictions or special meaning.


W3C References

HTML5

3.2.5.1 The id attribute

The id attribute specifies its element's unique identifier (ID).

The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character. The value must not contain any space characters.

Note: There are no other restrictions on what form an ID can take; in particular, IDs can consist of just digits, start with a digit, start with an underscore, consist of just punctuation, etc.

3.2.5.7 The class attribute

The attribute, if specified, must have a value that is a set of space-separated tokens representing the various classes that the element belongs to.

The classes that an HTML element has assigned to it consists of all the classes returned when the value of the class attribute is split on spaces. (Duplicates are ignored.)

There are no additional restrictions on the tokens authors can use in the class attribute, but authors are encouraged to use values that describe the nature of the content, rather than values that describe the desired presentation of the content.

The_Lone_Devil
20#
The_Lone_Devil Reply to 2016-10-06 13:55:41Z

A unique identifier for the element.

There must not be multiple elements in a document that have the same id value.

Any string, with the following restrictions:

  1. must be at least one character long
  2. must not contain any space characters:

    • U+0020 SPACE
    • U+0009 CHARACTER TABULATION (tab)
    • U+000A LINE FEED (LF)
    • U+000C FORM FEED (FF)
    • U+000D CARRIAGE RETURN (CR)

Using characters except ASCII letters and digits, '_', '-' and '.' may cause compatibility problems, as they weren't allowed in HTML 4. Though this restriction has been lifted in HTML 5, an ID should start with a letter for compatibility.

Tazwar Utshas
21#
Tazwar Utshas Reply to 2016-11-12 13:52:52Z

Any Alpha-numeric value and "-" and "_" is valid. But, you should start the id name with any character between A-Z or a-z.

Wembo Mulumba
22#
Wembo Mulumba Reply to 2017-05-09 04:22:32Z

No spaces, must begin with at least a char from a to z and 0 to 9.

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