Customizer: Allow arbitrary custom CSS

[sirreal]

• Allow arbitrary text in Customizer custom CSS.
• Protect the CSS data from mangling by KSES HTML filters.

Under some circumstances KSES would run post content filters and change the resulting content like this:

 @property --animate {
-  syntax: "<custom-ident>";
+  syntax: "";
   inherits: true;
   initial-value: false;
 }

This depends on #10656 to ensure styles output is safely printed in the HTML (merged here).

Trac ticket: https://core.trac.wordpress.org/ticket/64418

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[westonruter]

Just a couple trivial suggestions, but otherwise this looks great.

[sirreal]

#10641 includes some protection to ensure bad style tag contents are not saved (including </style> or ending in something that could become </style> after concatenation).

This change should be fine because of the work in #10656. Is it worth including that additional safety here?

On one hand, this change should be perfectly safe now.

On the other hand, there's not a very good reason to allow possible style tag closers inside of style tags. It will be safely escaped by the HTML API, but I wonder whether it's more helpful to prevent it from ever being stored.

[westonruter]

In other words, to restore the validate method but to change this:

if ( preg_match( '#</?\w+#', $css ) ) {

To be rather:

if ( false !== stripos( $css, '</style' ) ) {

Right? That seems fine to me. To be more conservative, and only make it even looser if we find this is actually needed.

[sirreal]

The check I've proposed is a bit more complicated. It also covers text that ends in partial style close tags <, </, … </style. This should ensure that even when concatenated with other text in a style tag, it cannot produce a style close tag. I do believe that these customizer styles are concatenated with other styles by the global styles system, so this check may be worth having.

The HTML API should still protect against broken style tag contents, so I'm not sure whether this protection is worth bringing here as well.

code sample

	protected function validate_custom_css( $css ) {
		$length = strlen( $css );
		for (
			$at = strcspn( $css, '<' );
			$at < $length;
			$at += strcspn( $css, '<', ++$at )
		) {
			$remaining_strlen         = $length - $at;
			$possible_style_close_tag = 0 === substr_compare( $css, '</style', $at, min( 7, $remaining_strlen ), true );
			if ( $possible_style_close_tag ) {
				if ( $remaining_strlen < 8 ) {
					return new WP_Error(
						'rest_custom_css_illegal_markup',
						sprintf(
							/* translators: %s is the CSS that was provided. */
							__( 'The CSS must not end in "%s".' ),
							esc_html( substr( $css, $at ) )
						),
						array( 'status' => 400 )
					);
				}


				if ( 1 === strspn( $css, " \t\f\r\n/>", $at + 7, 1 ) ) {
					return new WP_Error(
						'rest_custom_css_illegal_markup',
						sprintf(
							/* translators: %s is the CSS that was provided. */
							__( 'The CSS must not contain "%s".' ),
							esc_html( substr( $css, $at, 8 ) )
						),
						array( 'status' => 400 )
					);
				}
			}
		}


		return true;
	}