They have tons of plugins, tools and probably even paid tools to minify HTML, yet, all it takes is a single online form. This one has been performing best for me, for years now. They also have a couple other tools that are useful (if you work with JSON).
For CSS, there are just as many, yet I hadn’t found the ideal one so far. Between these three however I managed to minify anything without error:
The same https://www.minifier.org tool as above for CSS also supports JS.
However this one so far has been performing better for me in terms of JS:
This last tool (beautifytools.com) also has a bunch of other similar tools, from beautifiers to converters, it is quite the pool of tools.
General Formatting, and more
FreeFormatter is the place to go if you need to format about anything. It has formatters, escapers, generators, minifiers and so on, and mostly they work just fine.
Convert about any file type to any file type
If you need to convert a Word File to a PDF, or about anything you can imagine to anything else, for free, check out: