When you start creating a website, you have to make some choices before we start. Some of these choices are using HTML or XHTML, Tables or Divs, Floats or Position, Inline Styling, etc.
Doctype
Every website needs to have a doctype declared; failing to do so may result on your page rendered ineffectively. The best way is declaring a doctype like xhtml transitional, strict or html 4.01 and help meet standard requirements. As a personal experience, this has been an issue in SharePoint implementations, since default masterpages do not include a doctype. In a customization, the best way to deal with this is declare a doctype so you can fully transform your site.
External Stylesheets
you shouldn’t have to use the style tag for SEO purposes. Not the end of the world, but it is bad. You should always link to an external stylesheet or stylesheets. For larger projects, you would be better off using a set of stylesheets for different parts of the site, instead of one 10000 lines long stylesheet. It happens; especially of you need to apply hacks for different explorers.

