It occurs to me that it might be a good idea to have the pages on my site be modular... Is there some way I can just include (like I would in php) a file for my banner, navigation, footer, etc. I'm currently going through and editing every file on my site - all because I decided to make a change to my nav menu. If I have to go with php pages instead, I can do that, but I thought I'd check first.
If there is a way to do this, is it well-supported by all browsers (even old ones/IE/etc.)? If I have to go with php pages, are there SEO issues/complications to doing so? I would assume they would only look at the rendered pages, right?