Run Mix on a variable?

Maybe a bit misleading but here is what I got. I just changed to tailwindcss and that seems to require laravel mix purgecss.

My website is a website builder, once the website is build javascript will take the HTML and post it in a variable via request.

At the controller level I take that variable and save it to the database in a filed called PageHtml.

What I cant figure out, How can I take the variable that contains the page HTML and run it through laravel mix exporting the css to a variable or file. Ideal will be to return the css to a variable so I can also save it in the database with the page.