I have experience with file locking files. Therefore, I share this in the hope of helping, but you should know that this solution to this problem is a little complicated and not as straightforward as you want. Ask yourself these three questions:
First: the priority of your content?
Second: do you use WordPress?
if you do
Three: Are you sure you haven't installed some interfering plugins?
First: HTML5 semantics for prioritizing content are important, but they are likely to be erroneous. For example, one item, such as a sidebar, may be visible above the contents of the fold, but it is not considered the above. You know that the locking tool is blocking: the download page requires something that is not there at the right time, and it should wait. This unwanted element can lead to this situation.
Second: if you use WordPress and you bought a theme for it, then your theme is probably not standard. The problem can be solved, but not worth it, it takes time, and you should think about the next updates to the topic. Changing your theme is the easiest, I think, but a fix is possible. But if you use some static pages, just call the CSS files as soon as possible.
Three: Recently, I had such a problem for all my CSS and JavaScript files. This was due to the use of the plugin for lazy loading of images. He ruined everything, I deactivated it, and the problem was solved. It delays the loading of images and gets some advantages, but delays the loading of all pages to load all the files, more than the Cons! Go through your plugin for something that might affect.
I still have one blocking block, but I'm working on it. I use a plugin to merge CSS files, and the reason for this is blocking the output. I eliminate as many plugins as I can and create their abilities in a child theme to reduce the request for CSS files, then I can deactivate this merge plugin.