Old browsers didn't support the HTML placeholder attribute. For a number of years, `<label>` elements have been used in a few places in WordPress to emulate placeholders. It's time to improve semantics and interaction, use real placeholders when possible, and clean up some JavaScript.
- Quick Draft widget: it now uses visible `<label>` elements and a real `placeholder` attribute
- removes the related JavaScript
- Post title:
- keeps the "prompt" label for backwards compatibility
- improves the JavaScript to make the "prompt" label stay visible on focus and disappear when typing, like real placeholder do
- changes the post "prompt" text from "Enter title here" to "Add title" for consistency with the Block Editor
- cleans-up some CSS
Props Cheffheid, afercia.
Fixes#42390.
git-svn-id: https://develop.svn.wordpress.org/trunk@44896 602fd350-edb4-49c9-b593-d223f7449a82
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