Adherence to Web Standards set by the browser development community are essential to providing reliable, solid design that works across multiple platforms and clients. Cross-browser Compatibility, to me, is also a promise not to exclude users from the enjoyment of a site for their choice of browser.
I use Free and Open Source Software as much as possible in designing, programming, and hosting websites. Apart from the obvious cost benefits, free and open source (FOSS) projects tend to be more versatile, standards compliant, and reliable than proprietary software.
Usability not only means keeping a site free of annoyances like popup windows or small text, but being as helpful as possible with site and page structure, programmatic elements, and presentation. The only thing worse than ugly design is lazy design.