Wrap defined term in dfn elements

[Jym77]

Our definitions usually contain the defined term wrapped in underscores (hence in italic), e.g.

A _semantic link_ is any element which has the [semantic role][] of [`link`][link] or a [semantic role][] that inherits from the [`link`][link] role.

We should rather wrap them in (semantically correct) dfn elements (if our markdown processing allows it), e.g.

A <dfn>semantic link<dfn> is any element which has the [semantic role][] of [`link`][link] or a [semantic role][] that inherits from the [`link`][link] role.

We should also add an id to the dfn element either all the time or at least for those definitions that define more than one term (e.g. Text Inheriting its Programmatic Language from an Element so that we can refer the correct one when needed.

[Jym77]

Actually, this could also be used for names of listed conditions and would greatly help referring to them in the running text:

… at least one of the following condition is true:
- <dfn id="cond-foo">Foo</dfn> Lorem Ipsum Foo; or
- <dfn id="cond-bar">Bar</dfn> Lorem Ipsum Bar.

(…)

This element matches the [foo][#cond-foo] condition.

[WilcoFiers]

What is the benefit you expect to get from doing this?

[Jym77]

Using the semantic dfn element in the web pages, rather than the semantically meaningless em.
Plus, the possibility to give an id to these means we can refer to them directly which is useful for definitions that include several terms (or for the named list conditions).