replace id atribute with class for calendar_wrap value of the Calendar Widget

[alexvorn2]

I think the echo '<div id="calendar_wrap">'; should be replaced with echo '<div class="calendar_wrap">'; because having the same id on the pages is not valid, if we will have two calendars on the page.

[SergeyBiryukov]

Related: #16539

[transom]

Changes id="calendar_wrap" to class="calendar_wrap" in default widget

[transom]

The issues in #16539 are somewhat applicable in that it is possible that third-party themes are looking for the #calendar_wrap div. That said, the core themes do not address that #calendar_wrap. This change is not as critical given that the #calendar_wrap is not attached to an input field (or label) and unlikely to negatively impact screen readers.

The benefit is semantically correct markup. The negative is that a third-party theme may be looking for the #calendar_wrap

[nacin]

Normally we'd try to print #calendar_wrap on the first instance, and hide it from all subsequent instances. That way it's backwards compatible at least for a primary use case (one calendar, styled by a theme).

I think when adding a class name (which we'd do for all instances), we can probably come up with a better name in the process.

[MikeHansenMe]

Only show the id once.

[MikeHansenMe]

Refreshed with a strict check.

[wonderboymusic]

In 34381:

Widgets: add a static property to WP_Widget_Calendar to ensure that the id attribute is only output once.

Props MikeHansenMe, wonderboymusic.
Fixes #24604.