CSS selector with period in ID

The HTML spec allows for periods (.) in an id:

<img id="some.id" />

However, using a CSS ID selector rule will not match correctly:

#some.id { color: #f00; }

The CSS spec for ID Selectors does not mention this case. So I assume it is using the combination of a tag name and class selector? For example, a CSS rule of a.className would apply to all anchor tags (<a>) with a class name of className, like <a class="className"></a>.

Is it possible to have an external CSS file rule that references an HTML element by its id that has a period in it?

I expect not since the CSS spec specifies that a CSS "identifier" does not include the period as a valid character. So is this a fundamental mismatch between HTML and CSS specs? Is my only alternative to use a different type of CSS selection? Can anyone smarter than I confirm or deny this?

(I would remove the period from the HTML id attribute to simplify things, but it is a system-generated id, so I don't have the ability to change it in this case.)

After digging through the specs some more, I found the CSS spec does allow for backslash (\) escaping like most languages.

So in my example, the following rule would match:

#some\.id {
  color: #f00;
}

You could also use the attribute selector like this:

[id='some.id'] {
  color: #f00;
}

Since you are using id, you can also use document.getElementById() which checks the id value as a normal string and not a CSS selector. e.g. the following works too:

const myElem = document.getElementById("some.id");

The only drawback is, you can't limit the scope of search, like you could with querySelector e.g. someElement.querySelector(). but since Ids should always be unique, a document wide search with id is valid.