@Robk-Blue ,
Notepad++ uses a library called “Lexilla” (written and maintained by the Scintilla team) to handle syntax highlighting. They have chosen not to support CSS embedded in HTML. If they don’t, Notepad++ cannot (because Notepad++ uses the syntax highlighter “lexer” code from Lexilla without customizing it – trying to customize it would be a maintenance nightmare that the Notepad++ developers have rejected on multiple occasions.)
I will quote here some of what I wrote in a related GitHub issue a little over a year ago:
I confirmed that SciTE (Scintilla/Lexilla’s test-bed editor) doesn’t highlight CSS-embedded-in-HTML, either. It’s definitely up to the Lexilla project to implement embedded CSS, unfortunately. (I was hoping that there was a configuration that Notepad++ just wasn’t using, but alas, they don’t give us that feature)
I dug through the old requests at the Scintilla project (the Lexilla library used to be part of Scintilla as a whole, but it’s been split out to its own project, though it’s still run by the Scintilla organization). There, I found this 2007 request for for embedded CSS, and following some of the links, I’ve found it’s been a common request – and later replies, like this one have indicated that unless someone else implements it or pays the Lexilla team to implement it, it’s not going to happen (since the same individual is involved at the separated Lexilla library, I am assuming their position on embedded CSS is still the same). So, per the official Scintilla/Lexilla replies, it’s technically difficult and highly unlikely to ever happen, so I’d call this one “infeasible” to add to Notepad++.
Since Lexilla doesn’t have it, and no one has volunteered to implement it for them, Notepad++ is unable to support highlighting of embedded CSS inside HTML, which is unfortunate. (Someone once suggested, though I cannot find it either here or in GitHub, that we should just “see” that there’s embedded CSS, and fork out the CSS lexer instead of the HTML lexer for that part; unfortunately, the Lexilla library doesn’t provide that feature, and it would be hugely complicated (and maybe not at all feasible) for Notepad++ to try to provide that overtop of the Lexilla lexer. Which is unfortunate, because that would solve all such problems.)
