TASK: Remove special handling of `content` attribute in html-tags

The special rule for-content attributes in html-tags is removed. The only supported way to define content of html-tags is creating children that can be a single expression or a list of nodes or expressions.

Background. The special handling of the content-attributes always was was a bit strange and since the latest changes allow child-content to be defined as a single expression instead of an array aswell it is
no longer needed.