Adel of Pixeldima contacted me to look into a problem with some of the styles not being applied after autoptimization on the forum-page in his beautiful OKAB-theme. I had seen similar problems before, but decided to dig in this time to understand the root cause. That root cause turned out to be 2 CSS-files, one overriding the style of the other, having a different media-type, which messed up the load order once autoptimized. This can either be fixed at the theme-level, at AO configuration level or with a couple of lines of code hooking into AO’s API.

The problem

Without Autoptimize (which one can trigger by simply adding ?ao_noptimize=1 to the URL), these 2 relevant files were loaded (among many others obviously):

• bbpress.css (the default bbpress css-file) was loaded first with media=”screen”
• okab/bbpress-styles.css (part of the theme) is loaded second with media=”all”, overriding some of the CSS in bbpress.css

Autoptimize honors the media-types, resulting in 2 Autoptimized CSS-files;

• autoptimize_1a002577cefe349767025a16201f37a9.css with media=”all” is loaded first (because the very first CSS-file encountered in the HTML had media=”all”) which contains okab/bbpress-styles.css
• autoptimize_8e3c7dac90177214b6583286ddaa141f.css is loaded second with media=”screen”, containing bbpress.css

So due to the mediatype the code in bbpress-styles.css was loaded before the CSS in bbpress.css when Autoptimized, resulting in styles being overwritten the wrong way (bbpress-styles.css is overwritten by bbpress.css instead of the other way around).

The solution(s)

As Adel was the theme developer, he could immediately attack the root issue by enqueuing bbpress-styles.css with media=”screen”. This way both files share the same media-type and go in the same autoptimized CSS-file, resulting in the CSS of bbpress.css getting properly overwritten by the CSS of okab/bbpress-styles.css.
If you’re not the theme developer you’ll probably not want to change the theme’s code (although you could dequeue and re-enqueue in a child-theme’s functions.php), in which case you have two workarounds at your disposal: the easiest workaround being simply excluding the CSS-file that gets overwritten from CSS optimization (i.e. in this case simply adding okab/bbpress-style.css to the list of CSS optimization exclusions). Easy indeed, but on the downside you’ll have one non-optimized CSS-file. That’s why it’s a good thing we have our precious little API, allowing us to override the media-type of the offending file with just a couple of lines of code, like this;

add_filter('autoptimize_filter_css_tagmedia','check_mediatype',10,2);
function check_mediatype($media,$tag) {
  if ( strpos($tag,"okab/bbpress-styles.css") !== false ) {
	$media=array("screen");
  }
  return $media;
}

Conclusion

As Autoptimize honors the original CSS’s media-types, 2 CSS-files that ought to be loaded in a specific order might end up being loaded in the wrong order if their media-type is different. This can be fixed in the theme or worked around by excluding a specific CSS-file or using AO’s API to override the media-type for the offending CSS-file.