https://purgecss.com/ does this, kind of -- it used to be recommended by Tailwind back when Tailwind shipped a giant zip-bomb stylesheet of every possible property/value combination by default. I don't think it does the more complicated browser-like analysis you mention, though; it might just check whether class names appear in your HTML using a regex search.

The AMP WordPress plugin also does something like this IIRC (to try and fit stylesheets into AMP's size limit) but the tooling for it might be written in PHP.

How do you remove the unused CSS?

https://purifycss.online/

Above is a nice online version of Purify. But it just seems to minimize the CSS, and doesn’t delete the unused CSS.

I wrote that for my company ~3 jobs ago, except instead of working only on static HTML, it would: for a small percentage of our traffic loop in the background processing a couple CSS selectors at a time, adding CSS selectors that matched to a bloom filter that would be occasionally POSTed back to the server. Then I'd parse through our logs, pulling out the bloom filters, and comparing them to our CSS rules to find unused rules. It wasn't perfect so it required manually checking before deleting CSS rules but it went a long way towards flagging potentially unused CSS rules.

This would work nicely with static HTML, indeed. But once you have some JavaScript i.e. dynamic HTML, it won't work reliably anymore. Worse, it might even give you a list of manually curated CSS properties to "allow list".

Waaay back there used to be a Firebug plugin that would monitor which CSS was ever used as you interacted with a page. Worked great for exactly these dynamic pages - I was using it with React for a few years before Quantum killed the XUL addons.

I'd forgotten about it and am now wondering if anyone made a replacement...

Chrome Dev tools has this feature. It's not as useful because when you navigate to another page, it resets the used CSS rules.

this is incorrect.

the only thing that does not work is generating the css class like:

var x = 'hase-';

var t = x + 'danger'

which is an antipattern. (it can even happen inside java, c#, whatever language you use and it is still an antipattern)

Can you please elaborate why it is an antipattern? I'm not sure I understand.

At least regarding tailwindcss, it checks your code to filter out unused css classes. So it is recommended to have full class names in the code instead of constructing them by concatenating strings. So if you have a variable `buttonColor` that can be `red` or `blue` it is better to do something like

    switch(buttonColor) {
        'red': return 'button-red';
        'blue': return 'button-blue';
    }

over

    return 'button-' + buttonColor;

if you are in a big project and want to refactor classes the string concatenation pattern gets extremely painful, it’s even worse when it’s done on the server and on the frontend. At some point you will have trouble removing your classes. My company did this a lot and know we struggle to clean it up since basically all classes are used somewhere. What we do is basically use purgecss and than diff it and look for all occurrences that got removed which will take us a few months.

Oh ok it was about concatenation, not just this example case. Got it, thank you!