by Joseph Rézeau.
Thanks for your reply, Dan, and for pointing to the tracker issues.
Finally found http://docs.moodle.org/dev/CSS_coding_style
What is strange is that it says "Avoid using IDs as selectors wherever possible." but all of the examples of selectors it gives are in fact IDs, not classes.
In MDL-39517 I found a reference to https://github.com/csswizardry/CSS-Guidelines which I find quite interesting.
Let's have complete Moodle CSS guidelines ASAP.
Joseph