Is it possible to select elements in CSS by their HTML5 data attributes? This can easily be answered just by trying it, and the answer is, of course, yes. But this invariably leads us to the next question, 'Should we select elements in CSS by their HTML5 data attributes?' There are conflicting opinions on this.
In the 'no' camp is (or at least was, back in 2014) CSS legend Harry Roberts. In the article, Naming UI components in OOCSS, he wrote:
It’s important to note that although we can style HTML via its data-* attributes, we probably shouldn’t. data-* attributes are meant for holding data in markup, not for selecting on. This, from the HTML Living Standard (emphasis mine):
"Custom data attributes are intended to store custom data private to the page or application, for which there are no more appropriate attributes or elements."
The W3C spec was frustratingly vague on this point, but based purely on what it did and didn't say, I think Harry's conclusion was perfectly reasonable.
Since then, plenty of articles have suggested that it's perfectly appropriate to use custom data attributes as styling hooks, including MDN's guide, Using data attributes. There's even a CSS methodology called CUBE CSS which has adopted the data attribute hook as the preferred way of adding styles to component 'exceptions' (known as modifiers in BEM).
Thankfully, the WHATWG HTML Living Standard has since added a few more words and even some examples (emphasis mine):
Custom data attributes are intended to store custom data, state, annotations, and similar, private to the page or application, for which there are no more appropriate attributes or elements.
In this example, custom data attributes are used to store the result of a feature detection for PaymentRequest, which could be used in CSS to style a checkout page differently.
Authors should carefully design such extensions so that when the attributes are ignored and any associated CSS dropped, the page is still usable.
TL;DR: Yes, it's okay to use data-* attributes in CSS selectors, provided the page is still usable without them.