Less does not put any restrictions on used selectors/properties/values as long as they meet the base CSS grammar, e.g. the following Less code:
whatever {
whatever-else: I dont care;
&:tlhIngan(Hol)[DaH="mojaq-mey-vam"] {
color: pitch black #2;
}
}
results in this CSS:
whatever {
whatever-else: I dont care;
}
whatever:tlhIngan(Hol)[DaH="mojaq-mey-vam"] {
color: pitch black #2;
}
Additionally there's special escaping syntax to pass just any characters through (i.e. ~"whatever-'characters'-here//**\@\$^%\n"
).
So to answer the question: Yes, Less supports custom elements as well as any future standard elements unless they introduce a new incompatible syntax.