Responsive design and hi-dpi images

True and correct XHTML5 markup

WCAG, ATAG: Meeting of accessibility guidelines in full.

Tableless CSS markup, with no hacks

Support for all major web browsers

Inbuilt tools for checking webstandards conformance of XHTML5, CSS, and JavaScript

Extra markup semantics including Dublin Core support, schema.org, Open Graph, and microformats.

Standards-based JavaScript (modern DOM and AJAX, no DOM-0 or innerHTML)

Automatic cleanup of bad XHTML5: HTML outside your control (e.g. from RSS) will be cleaned up for you.