CSS Selector for selecting an element that comes BEFORE another element?

Question

I'm working on a login page for my website and I want to bold the label that comes before the input box when the text box gains focus. Right now I have the following markup:

<label for="username">Username:</label>
<input type="textbox" id="username" />

<label for="password">Password:</label>
<input type="textbox" id="password" />

I can use the adjacent selector to select the label after the text box, but I can't select the element before it. I can use this CSS selector rule, but it only selects the label after, so when the username text box gains focus, the password label becomes bold.

input:focus + label {font-weight: bold}

Is there anything I can do to make this work? I know JavaScript could be used to easily do this, but I'd like to use a purely CSS-based solution if possible, I just can't figure out a way to do it.

Answer 1

The CSS Selectors 4 Spec provides a syntax for defining the "subject" of a selector by using a !. As of early-2016, this is not available in any browser. I'm including this because it will be the correct way to do this using pure CSS once browsers implement this syntax.

Given the markup in the question

<label for="username">Username:</label>
<input type="textbox" id="username" />

<label for="password">Password:</label>
<input type="textbox" id="password" />

This CSS would do the trick.

label:has(+ input:focus) {font-weight: bold}

Of course, this assumes browsers actually implement the subject selector and that there are no bugs related to how this syntax works with the :focus pseudo-class.