For octane, we could check that an element is visible and contain some text, but the problem is the way wait_for_xxx is made is a while loop which does a sleep 0.5, and check the predicate again.

We should not run any code aside to the benchmark, because we don't want to perturb the benchmark while it is running.

The only solution which need some marionette instrumentation would be to let marionette listen for changes reported by the content, such as a  console.log("Marionette: Wake up!")  added by marionette before running the benchmark.  In kraken and sunspider benchmark (that I will submit later), the problem is different, as the page is changed, so we could add a hook on the window.url modification.