no CSS error
cannot use letter-spacing property, because of missed unit
a) update Wave library (used version from 2014: 0.6.0, newest version from 2018: 0.7.6)
b) fix this error with a unit (f.e. "rem")
c) check if this library is still in use and maybe do not use it anymore
All of this is a part of materialize.css and I think this is the line causing the error:
(the waves thing is just in the same line when minified)
As materialize.css is unmaintained, I doubt this will be fixed, but you could propose a fix to the fork (https://github.com/materializecss/materialize) and once Matomo uses the fork (https://github.com/matomo-org/matomo/issues/16368), this will be fixed.