It’s a bit hard to explain, but bare with me.
Does the style.css show up on the merged css specific file, under the status page, when you click to see the logs?
Have you tried to purge the cache on FVM?
When you disable css processing, you are right, nothing is done with CSS.
But when you enable it, it merges them into one file.
Unless your style.css is not being enqueued properly (ie: manually added to the header.php won’t work), I am almost certain that the style css is being merged properly, even with yoast enabled.
But you may be experiencing a conflict with too many css rules when merging css files, where Yoast is overwriting your style.css code by specificity.
You have only a few options here:
a)
https://wordpress.org/plugins/fast-velocity-minify/#faq
Under “After installing, why are some images and sliders not working?”
You could exclude some css files via the ignore list, such as style.css or yoast css code.
b)
You could try to enable the inline css on the header and footer option.
c)
You could try to use the autoptimize plugin and see if it works for your site.
If it works, I suggest sticking to it since it’s easier that way.
d)
Contact me via fastvelocity.com and send me your url, with a reference to this post.
Regards,
Raul
