I used to serve my pages declared as XHTML from free webhosts (remember those loads of 100-ish MB hosts that kept springing up in the 2000s?) that had the added

I used to serve my pages declared as XHTML from free webhosts (remember those loads of 100-ish MB hosts that kept springing up in the 2000s?) that had the added side-benefit that the injected banner adverts were not valid XHTML and therefore stripped by browsers at the time.