Switch to theme_boost Open its config file and comment $THEME->csstreepostprocessor = 'theme_boost_css_tree_post_processor'; Swtich to RTL and confirm that the blocks, user menu, etc... are flipped to the opposite side compared with LTR Uncomment the line above and refresh Confirm that the styles remained flipped for RTL Confirm that the main rule for HR contains two box-sizing definition, one of which is prefixed for webkit (you'll have to inspect the CSS file) Switch back to English Confirm again that box-sizing is being prefixed.