I am using a class called "mysub-header" on my multisite installation with the following properties:
border-bottom: thin solid Silver;
This has been working for years. It works on the main site, see for example this page where it affects the sub-header "Over een uitspraak van Leo Trotski".
However, it does not work anymore on one of the other sites, e.g. on this page where the sub-header is "Over amateurs en professionals in de poëzie".
I noticed that classes in general still work, e.g. <p class="example"> and the problem is mostly with <div>.
Would you have any clue what's going on?