Are CSS selectors case-sensitive?

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])

CSS4 (CSS Selector Level 4) adds support for case-insensitive match (ASCII only).

input[value='search' i]

It's the "i" at the end which would do the trick...

Check my other answer for details which browser supports this.