I considered it a no-brainer. I was wrong.
I wanted to take along some files to read when I'm out of network reach or in flight mode. The files happen to be quite plain HTML files, no script, just a CSS file defining the simple layout, and that's it. So I copied the HTML and the CSS files onto my phone and tried to open them.
First attempt was from a file manager. It told me that it no idea what to do with an HTML file. I then tried to look for an "open file" entry in Chrome. Nope. Same in Firefox. Then I tried to force-open the files by using a file:///path/to/file style URL - and it actually submitted the whole string to Google as a search string, as if Chrome (and Firefox) had been lobotomized of the file: protocol.
Now I'm at a loss - is there no way to open a simple HTML file on an Android 13 phone?