Empty form action attributes (action="") are invalid in HTML5.

[jdgrimes]

There are quite a few forms in core which have an empty action attribute. This ​isn't valid in HTML5. The HTML5 spec permits them to be removed entirely, but discourages leaving them empty.

This might make a good first bug.

[voldemortensen]

Removed all empty action attributes.

[wonderboymusic]

In 31200:

In HTML5, the action attribute is no longer required. Remove this attribute when empty.

The admin HTML is served with the HTML5 doctype.

"The action and formaction content attributes, if specified, must have a value that is a valid non-empty URL potentially surrounded by spaces."
​http://www.w3.org/html/wg/drafts/html/master/forms.html#attr-fs-action

Props voldemortensen.
Fixes #30126.

[SergeyBiryukov]

[31200] broke adding widgets via Customizer: 30126.widgets.png​.

There's a closing </div> for an opening <form> in each widget's control template.

[SergeyBiryukov]

In 31226:

Customizer: Fix form tag replacement in WP_Customize_Widgets::get_widget_control() after [31200].

see #30126.