XML-RPC: Avoid a PHP notice in pingback_ping() method.

props jesin, simonp303.
fixes #29177.
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Add styling for various HTML5 input elements. props avryl. fixes #28262.

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Better focus styling for buttons and button groups. props mattheu, avryl, celloexpressions. fixes #27826. see #28267.

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Docblock corrections for `get_the_terms()` function and corresponding filter, and `registered_taxonomy` action. Fixes #29183.
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Correct the docblock for the `registered_post_type` action. Fixes #29184.
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Correct references to post-template.php in the inline docs.

props softmodeling.
fixes #29188.
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Clearer visual differentiation between panels and sections in the customizer. props celloexpressions. see #28979.

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