Java’s Collections.unmodifiableCollection doesn’t pass equals and hashCode to the backing collection. I’m surprised I haven’t come across this before!
The user-select CSS property allows you to prevent selection of certain content. As the link explains, it’s useful for preventing accidental selection of icons next to buttons. A certain well known UK high street electronics shop’s website also uses it to prevent text selection, presumably to prevent you copying product names into price comparison sites. It’s not reliable or widely supported though. The same effect can be had by doing a preventDefault (or return false) in the “onselectstart” event handler (again doesn’t seem to be standard).
Continuing on the theme of text selection, I find the auto highlighting of library URLs on hover on the cdnjs website quite useful. It’s achieved using the Selection capabilities of the DOM, which allows you to get or set the current selected text.