In this section I'll look at UK government recommendations scattered through the first half of the official accessibility document (up to section 2.4.3) that don't match up exactly with WAI level A or Double-A guidelines, and aren't included in the second half of the document (the half that's easier to follow).
This excludes any recommendations that don't appear to make sense - those are covered Questionable Elements.
The above rules may require some technical knowledge, but are otherwise relatively straightforward, so won't be discussed in detail. The ones covered below require more interpretation.
Any colour used must be customisable by the end-user.
What does this mean? It means specify your colours as HTML code, rather than using coloured gifs as backgrounds. If you're using colours merely for eye-candy, you don't need to worry about it. But there are occasions when you might use colours for text and backgrounds, and then you need to use code rather than images so that if your visitors have some kind of colourblindness they can change the hues to give a contrast that they are capable of seeing.
A consistent text navigation bar should be used along with a 'skip navigation
Provide 'skip navigation links' link at the top of each page containing the main menu buttons.
What's a skip navigation link? It's a link to an HTML anchor just below the navigation bar, where the real content starts. Press it and the screen scrolls you a couple of centimetres down the page. It's useful for people using screen readers (talking browsers) because it allows them to skip repeated navigation that would otherwise waste a lot of their listening time.
But for regular sighted users it can be confusing, because it doesn't seem to accomplish much. The answer is to make it invisible, and this is how it's dealt with in the pages of the E-envoy site (also here on Tinhat).
People who are visually or physically impaired may require keyboard equivalents
for (these) mouse actions.
Many people cannot use a mouse but must tab through content to get to the options or information they need on a page.
This means use 'tabindex' where necessary, for example in complicated forms, avoid mouse-specific event handlers, for example OnMouseDown, and include accesskeys - which are covered in a second-half section of the guidelines.
Provide clear and consistent navigation mechanisms - orientation information, navigation bars, a site map, etc in order to increase the likelihood that a user will find what they are looking for on your site
In the guidelines section immediately before this, the recommendation is "Provide context and orientation information to help your users understand complex pages or elements." So we can infer that perhaps the same applies to site maps, and that they should be used when there is a need, but that they are not mandatory.
Splash screens and accessibility
The guidelines strongly discourage the use of Splash screens, meaning opening pages that are created for visual impression and contain no content (the content arrives on a second page). Most usability experts would take a stronger view, beyond mere discouragement, and say categorically that they must not be used.
Always specify whether the font is to be sans-serif or serif as the lowest default setting
The smallest sans-serif text is usually more legible than the smallest serif. So this guideline is there to avoid small text turning out as serif.
This Tinhat page is valid XHTML to WAI Level-A standard
UK accessibility intro + menu
UK accessibility site reviews
Comment on DRC Report
Simple WAI level A checklist
Level A -
TinHat Level A+
Tips for web editors
Non-HTML files (PDFs) and accessibility
Gov guidelines for UK gov sites (intro)