The following method in class.wp-scripts.php could break your css if you have strict standards enabled on your server.

	function set_group( $handle, $recursion, $group = false ) {
		$grp = isset($this->registered[$handle]->extra['group']) ? (int) $this->registered[$handle]->extra['group'] : 0;
		if ( false !== $group && $grp > $group )
			$grp = $group;
 
		return parent::set_group( $handle, $recursion, $grp );
	}

This is because the WP_Scripts inheritance from WP_Dependencies does not follow the signature of the method. WP_Dependencies have the set_group function declared without any default parameters while the subclass have defaulted $group to false.

Easy fix! Just remove the default value on the group parameter so it looks like this:

	function set_group( $handle, $recursion, $group ) {
		$grp = isset($this->registered[$handle]->extra['group']) ? (int) $this->registered[$handle]->extra['group'] : 0;
		if ( false !== $group && $grp > $group )
			$grp = $group;
 
		return parent::set_group( $handle, $recursion, $grp );
	}

The only place I can find this possible being called from send in a value anyway so there’s no need for having the default value as of right now.