Removal of HTML5 script theme support affects older theme gallery layout in frontend

[Ov3rfly]

This is a follow-up to #64442.

On a website with older theme ​Renkon 1.0.9 from ElmaStudio the WordPress gallery shortcode causes layout issues after WordPress 7.0 update.

Reason is the removal of gallery shortcode inline CSS by the theme which does not work any more due to missing type attribute in WordPress 7.0 $gallery_style code:

/*-----------------------------------------------------------------------------------*/
/* Remove inline styles printed when the gallery shortcode is used.
/*-----------------------------------------------------------------------------------*/
function renkon_remove_gallery_css( $css ) {
	return preg_replace( "#<style type=\"text/css\">(.*?)</style>#s", '', $css );
}
add_filter( 'gallery_style', 'renkon_remove_gallery_css' );

Out hotfix was updating the older theme code, so their filter works again.

[sabernhardt]

WordPress 3.1 introduced a more reliable filter than gallery_style for removing the style tag completely.

add_filter( 'use_default_gallery_style', '__return_false' );

Twenty Ten has both filters, keeping gallery_style for backward compatibility with WordPress 3.0.

Several other ​themes in the directory have a gallery_style filter, and most of them replace
<style type='text/css'>(.*?)</style>. If they do not also have the use_default_gallery_style hook, I expect that those themes already had a problem in WordPress 5.3, when [46164] switched the quotes from single to double.

cc: @elmastudio