Currently forumng message span the available width of the screen. This causes usability and accessibility problems for users who maximise their browser on a large monitor. In order to resolve this, we will set the maximum width of a post to 800 pixels (the general minimum screen width)

In other words, the left margin of the post is unchanged, but the width of the post should be a maximum of 'some value'. This means that the first few levels of indent have a fixed width, while later levels are the same as before (because they were already close to the right margin). This can probably be done just with CSS max-width.