Check for specific content and fire when ready.

fireWhenReady2.js

function callWhenReady(testCB, readyCB) {
  console.log('checking for page ready');
  var i = setInterval(function(){ // after 1/4 of a sec
  	  var res = testCB();
      if (res === true){ // wait X sec if node count does not change, page likely finished loading.
              clearInterval(i);
              readyCB();
      }
  }, 250);
}
callWhenReady(
    () => { //test queries
	    return !!document.querySelector('#sxm-header ~ .program-schedule #canadian.channel-genre') || //www.siriusxm.com/programschedules
	           !!document.querySelector('body[data-shorten-url="//www.foodnetwork.com"] #dfp_bigbox iframe');
	},
    () => { //when any query above is true do this
        alert("DOM is ready !!! ");
    }
)