With a clean moodle install go to these pages and make sure they validate at validator.w3.org: Administration block -> Site Administration -> Users -> Permissions -> Site administratios Click on calendar month in calendar block -> Create new event Also, try this pages and make sure they have no "summary" attributes on tables Administration block -> Site Administration -> Users -> Permissions -> Assign system roles -> Choose a role (it does not validate because "The cellspacing attribute on the table element is obsolete.") Cohort assign page: Create a new cohort at system level. Go to "Assign cohort page" Make sure it has no "summary" attribute on tables (it does not validate because "The cellspacing attribute on the table element is obsolete.") Group assign and grouping assing page: Create a course Create a group in this course Go to "Add/remove users" page and make sure it validates Create a grouping Go to "Add/remove groups" page and make sure it has no "summary" attribute on tables (it does not validate because problems with valign obsolete attribute, cellspacing also obsolete and an empty action attribute)