Rephrase and reformat for more brevity and clarity.
Fix typos, add crosslinks.
Less gaudy colors in the element hierarchy diagram.
Make the "Element Reference" introduction a sub-section,
so it turns up in the local ToC.
Update analogies to HTML (now referring to HTML5 rsp. the "living standard").
Fix analogy to `<h1>` to `<h6>`: these mark titles (headings), not sections.
Reduce redundancy: Only list including "Parameter Entities" if not
implied by the element category).
Update description of `<line-block>` (it used to be a
sort of literal-block but is is now rather a plain list,
cf. docs/dev/rst/alternatives.html#syntax-for-line-blocks).
Small section for "placeholder" parameter entities.
Move the description of "Pseudo-XML" and the "Names and identifiers"
section content to a new "Glossary" section.
Bibliography entry for docutils.dtd.