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ID selectors
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The CSS ID selector matches an element based on the value of the element's id attribute. In order for the element to be selected, its id attribute must match exactly the value given in the selector.
css
/* The element with id="demo" */
#demo {
border: red 2px solid;
}
Note that syntactically (but not specificity-wise), this is equivalent to the following attribute selector:
css
[id="id_value"] {
/* … */
}
The id_value value must be a valid CSS identifier. HTML id attributes which are not valid CSS identifiers must be escaped before they can be used in ID selectors.
HTML
html
<p id="blue">This paragraph has a blue background.</p>
<p>This is just a regular paragraph.</p>
html
<!-- The next two paragraphs have id attributes
that contain characters which must be escaped in CSS -->
<p id="item?one">This paragraph has a pink background.</p>
<p id="123item">This paragraph has a yellow background.</p>
CSS
css
#blue {
background-color: skyblue;
}
css
/* In the next two rules, the id attributes must be escaped */
#item\?one {
background-color: pink;
}
#\00003123item {
background-color: yellow;
}
Result
The ID selectors in the following rules are not valid CSS identifiers, and will be ignored.
css
#item?one {
background-color: green;
}
#123item {
background-color: green;
}