If you search the web for information related to web accessibility for deaf people you will find plenty of advice about captioning or providing transcripts for web based audio and video material. What you are unlikely to find is much discussion related to accessibility and language; for many deaf people English is not their first language, Sign Language is.
Although Sign Language provides an equivalent for everything that can be spoken or written, understanding written English – for some deaf people – is a process of interpreting from English to their first language, i.e. Sign Language.
Writing simple language and short sentences can help to make information more accessible to Sign Language users. However having discussed the issues with various informed users in the past (e.g. those at the Sign Language Interpreter Service in Glasgow) it seems that the most effective way to make content accessible to Sign Language users is to provide a Sign Language version of all content.
The problem here is that the obvious way to do this, i.e., providing video of Sign Language interpreters, is an expensive and resource hungry exercise. For this reason, many people are experimenting with signing avatars (virtual humans) as a way to deliver Sign Language equivalent to written content.