cross browser - Are CSS selectors case-sensitive?

Question

I was recently updating a CMS site and a tab-navigation plugin had inserted the following markup:

<li id="News_tab">...

I've always written my CSS selectors in lowercase so when I tried to style this with #news_tab, it wouldn't apply, but #News_tab worked.

After all these years I'm surprised that I haven't run into this before, so I've always been under the impression that CSS was case-insensitive. Has CSS always been case-sensitive and I just haven't noticed thanks to my consistent code style?

Answer 1

CSS itself is case insensitive, but selectors from HTML (class and id) are case sensitive:

CSS recommendation on case sensitivity

HTML recommendation, id attribute (note the [CS])