All content should ideally be in Plain English. See http://www.plainenglish.co.uk for more information. In particular, the doc http://www.plainenglish.co.uk/files/howto.pdf is essential reading and http://www.plainenglish.co.uk/files/alternative.pdf is very useful too.

In summary, they recommend:

• Keep your sentences short
• Prefer active verbs
• Use 'you' and 'we'
• Use words that are appropriate for the reader
• Don't be afraid to give instructions
• Avoid nominalizations
• Use lists where appropriate

Some other points that I picked up from working briefly with a representative from the Plain English Campaign some time ago regarding accessibility:

• People find lower case letters easier to read than upper case letters, so wherever possible use sentence case, including for titles. Proper nouns need to be initial capped and the council has a lot of these (names of plans and so on).
• Avoid red text as people find this difficult to read.

From my experiences with Simply Understand, other websites I've worked on and training I do at work, I would add that it's good to:

• Present instructions in list form so you can do the steps you have to take to do something
• Integrate your urls into the text (see above!) so that they flow with the text and don't interrupt reading. Same with email addresses.
• Use tables to present data
• Bold key words and concepts for emphasis
• neat descriptive headings
• Short, concise paragraphs (2-3 lines is a good guide)
• Re-enforce the point of the page at the top and the bottom.
• Left-align your text.

But try not to:

• Use italics, they're very hard to read.
• Use tables to present anything that isn't data
• Go overboard on the bolding

If you're not sure what you're doing is right, get someone else to read it.