There's an ongoing issue with adding a tag attribute to the input element. Changing it to data-tag gets rid of the HTML validation error but causes UI glitches.

error: Attribute tag not allowed on element input at this point.
From line 156, column 55; to line 156, column 163

See http://help.simplytestable.com/errors/html-validation/attribute-x-not-allowed-on-element-y-at-this-point/

Unacceptable {tag} attribute inside {input} tag
change 'tag' to 'data-tag' in the following instances

[/inc/core/ui/forms/_form.class.php]
Lines: 2940,3706,3710
Causes UI glitches.

[/inc/widgets/views/_widget_list.view.php]
Lines: 266,272,282,287,297,303
removes glitches in back office widget form:

[/inc/core/ui/forms/_uiwidget.class.php]
Line: 536
No obvious outcome.

[/inc/items/view/_item_list_sidebar.view.php]
Line: 86, 318
Sorted glitch in back office Posts:

See http://forums.b2evolution.net/html5-validation-of-6-7-4#c107291