Problem/Motivation
The template preprocess functions in form.inc for fieldsset and form element label do prepare the title variable by using '#markup', but the template preprocess funciton for details passes the title variable as is, which leads to different behavior of fieldsets, form elements and detaills, as an inline html inside a title string of a details element will be escaped and not rendered in the browser, but an inline html inside title of any other form element will not be escaped and rendered correctly in the browser.
For example, untranslatable fields are extended with a "all translations" hint to make sure the editor understands the affect of changing it:
Proposed resolution
Use #markup for titles of details elements.
Remaining tasks
None
User interface changes
None
API changes
None
Data model changes
None