I always try to make the theme which I have designed (both free and paid) compliant with web standard. You will see icons at right bottom of the theme. What is compliant with web standard mean? It means that the theme has been checked with Validation Service and it got valid CSS and XHTML. When you see Valid CSS and Valid XHTML icons, it shows that the theme is checked and no errors found in CSS and XHTML.
When you have the Valid CSS and XHTML icons on your weblog, it will show to your readers that you have taken the care to create an interoperable web page.
As professional designer, compliant with web standard is important.