Not to kid, but HTML.
It’s cross-platform, and sums up the gui-layout in a simple textfile. It’s definitely mature, as well as well-understood and well documented.
There’s a bunch of ways to template HTML files for dynamic content, and other ways to convert custom syntaxes to HTML if you dislike angle-brackets.
Client-side scripting w/ Javascript, server-side scripting with PHP/Ruby/Python/Perl.
It’s not well suited for all purposes, but for many, it’s good enough. There’s no reason that it has to be served either – you can distribute a HTML file to your clients if you want – see TiddlyWiki for a good example of where that can go.