Refactor fragments

CSS: Don't override font-weight for section

.slides>section and .slides>section>section set the font-weight to
"normal". This overrides any font-weight setting a theme may set at
the body or .reveal level, and requires a theme author to also
specifically set the font-weight for .slides>section and
.slides>section>section. That's tedious and also counterintuitive.

Adopt a saner default by setting the font-weight to inherit.

It should be `{. xxxx=yyy }`, not `{_ xxxx=yyy }`.
Only test/test-element-attributes-markdown.html were using `{_`,
simply in order to test the ability, for the user, to define a _different_ pattern.

It is important that test/test-element-attributes-markdown.html do test
the ability for the user to define its own pattern.