Several accessibility issues:

• The list of users with images, eg 'Users tagged with "Moodle": 42' should be marked up as an unordered list <ul> - use "inline-list" class,
  http://docs.moodle.org/en/Accessibility_notes#Pattern_1:_unlist.2C_inline-list
• As the user images are the only contents of a link they should NOT have empty ALT text - basic stuff!

I have a patch, tested on Win/Firefox 2.0.0.8 and IE 6, affecting 2 files:

• tag/lib.php, print_user_list (clue in the name) and print_user_box functions.
• theme/standard/styles_layout.css - a long overdue fix for "inline-list" class!

(Also see bug MDL-11430)