I noticed recently that it doesn't seem to combat the worst offenders: sites using width= attributes or invisible-table-based margins. I hit Readability, and the content is still squished to one side. Heck, I turn styles off and it's still squished to one side. This is what people mean when they say these old sites aren't "accessible": they can't be transformed into pure content to be input into something else (Readability, or a screen-reader, it makes little difference.)