Process and include external html file in build

I have a html file and css file in src directory. I want to process them at the time of compilation and include them in the build. I have seen the documentation for build.rs but I am not sure how can I process these files.
Any help regarding processing non .rs files?

Can you elaborate on what processing you want to do?

If it's just a case of embedding the files in your binary so they can be served at runtime, look into the include_str!() macro or the include_dir crate.

Thank you @Michael-F-Bryan. I need to include the CSS file in binary but also process html to convert it into another format.
Ex. There can be multiple html files, I want to combine all and include single html file. Same for CSS files also.
For this I first need to find all the html files present in the project.
Hope this clears the requirement.