* no mobile/smartphone support with a dedicated css (no mobile tag)
Appearance > extras, select "Display a mobile theme when this blog is viewed with a mobile browser" and click save.
* large screen support: most, if not all, themes seems to have the text column with a fixed sized which makes half of the screen empty on 24" or bigger display (no large display tag)
20% of internet users are still on 1024 x 768 monitor resolution or lower, and from a graphic designer and page layout standpoint, you don't want the text area to be over about 700px wide or it makes it very difficult for people to read. I have a 30" monitor and I NEVER set my browser that wide, even on flexible width sites. It is just too difficult to read. Also in my experience, relying on the browser to pass on the monitor, and even the OS information only happens about 50% of the time (I watch those stats). 50% are identified as "unknown". (There are currently 15 flexible width themes here. Use the "feature filter" at appearance > themes and select "flexible width."
Also, if you design for a variety of monitor widths (if the browser will even pass that information to you) What size do you make the header image if used? Do you make it for a 24" monitor and then if the people are on a 1024" wide monitor you just chop off the sides of the image? Or do you scale the image and squish it horizontally to something unrecognizable on a 1024px wide display? Navigation becomes a major issue as well since to accommodate a 1024px wide monitor, when displayed on a 24" monitor the tabs are either spaced out with huge amounts of space between them or they are all crowded over into the left or right corner. And what about those people on the 800 x 400 netbooks?
Trying to design a site so that it works well from a 800px to 2560px width, and retain any sort of balance and continuity of design is a monumental task (massive understatement) as you have to scale not only the main page elements, but also all the text and the widths of the sidebars, heights of the navigation and footer elements and virtually everything else.
Why do you need validation links at the bottom of your theme? I never include them in the footers of sites I design. If anything they will go on the about page. Virtually no one outside of techie geeks (such as myself) would even know what it means. If you mention W3C to 100 people 99 of them will think you are talking about professional wrestling.