Is that standardized? So why doesn't all those libraries (mostly jQuery libraries) that perform DOM transformation use custom elements, instead of a lot of nested divs with custom classes?
Honestly, I'm not sure. If it was really an issue, it'd be trivial to change the script to render things as nested divs and spans. The appeal of custom elements, at least to me, is that it makes everything more concise and readable. I prefer reading <type>whatever</type> to <div class="type">whatever</div>.