KG's 11 dimensional retrospection

Here are basic TO-DO list and Tips of improving Web page performance with modern way (well, as of Nov. 2010...).

Well, at least jQuery or YUI.
ASP.NET can use Ajax Control Toolkit

YUI CSS Reset
The foundational YUI Reset CSS file removes and neutralizes the inconsistent default styling of HTML elements, creating a level playing field across A-grade browsers and providing a sound foundation upon which you can explicitly declare your intentions.

HTML5 - HTML5 Boilerplate contains a very good essence of basic HTML/CSS structure.

YUI Exceptional Performance provide a Best Practices and tool (YSlow).

Google Chrome also provide a developer tools built in, which has a similar functionality as YSlow.

YUI Loader Utility
"Loading YUI: Seeds, Core, and Combo-handling", by Eric Miraglia
The YUI Loader makes it possible to load all necessary YUI library just by one call. You don't need to include various dependent files by yourself.

YUI 2: ImageLoader
Loading images only when those are visible.

Minify, Combine CSS and JavaScript with Visual Studio

MS' tool: Microsoft Ajax Minifier 4.0
Yahoo's tool: YUI Compressor

CSS Sprites: What They Are, Why They’re Cool, and How To Use Them
Instead of thousands of small images, combine them into one image, and use CSS to display necessary portion as icons.