Comment by ZYbCRq22HbJ2y7

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/if...

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/po...

https://en.wikipedia.org/wiki/Server_Side_Includes

Also, other than to satisfy a purist desire, why do you need this?

You can pre-render HTML quickly using a variety of template systems, and it would outperform what you are suggesting every time (from a client's perspective). I mean, think about the potential CSS resolution complexity, FOUC, etc.