0
	 */
	private function combine_rules_selectors() {
		// Build an array of selectors along with the JSON-ified styles to make comparisons easier.
		$selectors_json = array();
		foreach ( $this->css_rules as $rule ) {
			$declarations = $rule->get_declarations()->get_declarations();
			ksort( $declarations );
			$selectors_json[ $rule->get_selector() ] = wp_json_encode( $declarations );
		}

		// Combine selectors that have the same styles.
		foreach ( $selectors_json as $selector => $json ) {
			// Get selectors that use the same styles.
			$duplicates = array_keys( $selectors_json, $json, true );
			// Skip if there are no duplicates.
			if ( 1 >= count( $duplicates ) ) {
				continue;
			}

			$declarations = $this->css_rules[ $selector ]->get_declarations();

			foreach ( $duplicates as $key ) {
				// Unset the duplicates from the $selectors_json array to avoid looping through them as well.
				unset( $selectors_json[ $key ] );
				// Remove the rules from the rules collection.
				unset( $this->css_rules[ $key ] );
			}
			// Create a new rule with the combined selectors.
			$duplicate_selectors                     = implode( ',', $duplicates );
			$this->css_rules[ $duplicate_selectors ] = new WP_Style_Engine_CSS_Rule( $duplicate_selectors, $declarations );
		}
	}
}