Persistent.info's nice mini-tutorial on, and example of, customizing the look of Gmail provide some nice evidence of two things I'm a fan of:

• Using XHTML and CSS gives you a lot of flexibility, by coming a big step closer to separating content from presentation
• By encouraging a hacker culture, Gmail continues to experience innovation and improvement that more tightly-controlled web-mail apps like Hotmail and Yahoo Mail can muster

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