Many of them using Custom Theme with multiple custom modules. In custom module, we calling particular module css on Layout xml file.
When we enable Merge CSS file option under
admin -> Stores > Settings > Configuration > In the panel on the left under Advanced > choose Developer.
Currently, It merge all CSS in single file and loading the entire website. But, unwanted css (which not related to the page) also loading.
I am looking to merge the CSS on page wise and working on it. If anyone have an idea about this please comment it.