Can CSS ID Selectors Handle Periods in HTML IDs?

Addressing ID Selectors with Periods in CSS

In HTML, periods are permitted within id attributes. However, when using these elements in CSS ID selector rules, inconsistencies arise due to periods not being part of valid CSS "identifiers."

Some individuals assume that CSS ID selector rules follow the syntax of a tag name and class selector combination, such as "a.className" targeting anchor tags with the specified class name. This leads to the question of whether it's possible to reference HTML elements with IDs containing periods in external CSS files.

The CSS spec clarifies that periods are not part of valid "identifiers," suggesting they cannot be used in ID selector rules. However, further exploration reveals that CSS does support backslash () escaping for special characters.

Therefore, to match an HTML element with an id containing a period, the following escaped CSS rule can be used:

<code class="css">#some\.id {
  color: #f00;
}</code>

This syntax allows CSS ID selector rules to reference elements with periods in their ids, aligning with the HTML specification and offering a viable alternative to modifying the HTML id attribute.

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