This is a meta-issue for gathering all the requirements for Bartik, Busy and Corolla themes to get into Drupal 7 core.
This is just my personal proposal, feel free to discuss and extend it further :
- - print support
- - RTL support
- - all pages render properly under IE6/IE7/IE8 and standards-compliant browsers
- - valid XHTML 1.0 and CSS3 (with browser extensions allowed)
- - all files are structured consistently with other core themes
- - code follows Drupal coding standards
- - primary and secondary links are hard-coded in page.tpl.php (pointed by Joachim)
- - no custom theme settings (pointed by webchick)
- - consistent stylings for hover, focus and active states
- - region names consistent with other themes
I'm not sure whether following features should be required (more opinions needed):
- old color module support (if patch from this issue will not get into core) - being limited just to 5 colors means providing color schemes that will be plain ugly. Old color module can shift similar colors, but this feature produces disastrous results (probably because colors are shifted in RGB space)
- admin/overlay stylings - doing this properly is a bit tricky, even Seven theme which is an admin theme by design has a lot of issues. Currently Drupal assumes that all themes are supporting admin stylings - it would make more sense to provide a way for specifying whether theme should show up in list of admin themes and focus all the work on Seven theme.
- full WCAG AA compilance - just like with old color module, some portions of WCAG AA would require me to make the theme ugly, I mean especially color contrast requirements.
- handheld stylesheet - both Garland and Seven don't support it, mobiles these days are ignoring it anyway.