Why does ASP.NET webforms need the Runat="Server" attribute?

I usually don't like to guess, but I'm going to on this one...

If you remember Microsoft's .NET marketing hype back in the day (2001?), it was hard to tell what .NET even was. Was it a server? a programming platform? a language? something new entirely? Given the ads, it was ambiguously anything you wanted it to be - it just solved any problem you might have.

So, my guess is there was a hidden grand vision that ASP.NET code could run anywhere - server side OR client side, in a copy of Internet Explorer tied to the .NET runtime. runat="server" is just a vestigial remnant, left behind because it's client-side equivalent never made it to production.

Remember those weird ads?

Related: Article from The Register with some .NET history.

I've always believed it was there more for the understanding that you can mix ASP.NET tags and HTML Tags, and HTML Tags have the option of either being runat="server" or not. It doesn't hurt anything to leave the tag in, and it causes a compiler error to take it out. The more things you imply about web language, the less easy it is for a budding programmer to come in and learn it. That's as good a reason as any to be verbose about tag attributes.

This conversation was had on Mike Schinkel's Blog between himself and Talbot Crowell of Microsoft National Services. The relevant information is below (first paragraph paraphrased due to grammatical errors in source):

[...] but the importance of <runat="server"> is more for consistency and extensibility.

If the developer has to mark some tags (viz. <asp: />) for the ASP.NET Engine to ignore, then there's also the potential issue of namespace collisions among tags and future enhancements. By requiring the <runat="server"> attribute, this is negated.

It continues:

If <runat=client> was required for all client-side tags, the parser would need to parse all tags and strip out the <runat=client> part.

He continues:

Currently, If my guess is correct, the parser simply ignores all text (tags or no tags) unless it is a tag with the runat=server attribute or a “<%” prefix or ssi “<!– #include… (...) Also, since ASP.NET is designed to allow separation of the web designers (foo.aspx) from the web developers (foo.aspx.vb), the web designers can use their own web designer tools to place HTML and client-side JavaScript without having to know about ASP.NET specific tags or attributes.