[dom] Missing spaces in rendered spec HTML

cvrebert has just created a new issue for 
https://github.com/whatwg/dom:

== Missing spaces in rendered spec HTML ==
In a few places in the bikeshed source, this:
```html
<a>:scope element</a> <a>context object</a>
```
is getting compiled to the following HTML (do a View Source on 
https://dom.spec.whatwg.org/):
```html
<a data-link-type="dfn" 
href="https://drafts.csswg.org/selectors-4/#scope-element">:scope 
element</a><a data-link-type="dfn" href="#context-object">context 
object</a>
```

Note the lack of a space between the two resulting `<a>`s.

See https://github.com/whatwg/dom/issues/65