I'm hoping I can do this without adding code to the actual pages.<BR><BR>I use lots of SSI to keep things consistant, but just one CSS file.<BR><BR>I have a header SSI that is just a basic HTML table.

jsFiddle is the perfect solution; it provides a custom environment (based on user selections) to test (or fiddle with) your JavaScript, HTML, and CSS code right inside your browser. A quick tour ...

Now [Andy Green] has extended the possibilities with affordable silicon, by producing a proof-of-concept HTML + CSS renderer over h2 on ESP32 for libwebsockets. Surf the web on a microcontroller ...