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id attribute is used to identify a single element within a document, and is required to be unique. Check the document for repeated IDs.
Related W3C validator issues
The id attribute of an HTML element, used to identify the element when linking, scripting or styling, must be unique in the whole document and must not contain whitespace.
Technically, in HTML5, the value for an id attribute may contain any character, except whitespace characters. However, to avoid inadvertent errors, only ASCII letters, digits, _, and - should be used and the value for an id attribute should start with a letter.
The id attribute is used to identify a single element within a document, it’s not required, but if used it must be unique, and must not be an empty string.
IDs for HTML elements can’t be blank.
The <label> element represents a caption in a document, and it can be associated with a form input using the for attribute, which must be an ID. Document IDs cannot contain whitespace.
<form> <label for="user_name">Name</label> <input type="text" id="user_name" /> </form>
A <pattern> element has been found with an invalid ID. Check the format of the ID and ensure it does not start with a digit, full stop (.) or hyphen (-).
The <pattern> element is used within <svg> elements, which use XML 1.0 syntax. That syntax specifies that valid IDs only include designated characters (letters, digits, and a few punctuation marks), and do not start with a digit, a full stop (.) character, or a hyphen-minus (-) character.
Element IDs in an HTML document must be unique. The HTML validator is indicating the first occurrence of an ID that is repeated. Check the details for that issue to see web pages affected, and the elements within them, to fix that ID repetition.
In the days before HTML5, named anchors were used as a way to provide a link to a specific section of a document, for example:
<h2> <a name="section-5">Section 5</a> </h2>
Now in HTML5, the name attribute is obsolete for <a> tags, and instead, you can use the id attribute of any element (not just <a>) as a way to navigate directly to it, for example:
<h2 id="section-5">Section 5</h2>
The <option> element no longer accepts a name attribute, which is now obsolete.
<select id="pet-select"> <option value="">--Please choose an option--</option> <option value="dog">Dog</option> <option value="cat">Cat</option> <option value="hamster">Hamster</option> </select>
<label> tags are used to label inputs in form, which need to be present and visible in the document, for example:
<label for="age">Age</label> <input id="age" />
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