New WebPageTest Experiment: Edit Response HTML!

We're happy to announce the addition of a new custom experiment in WebPageTest Pro! The new "Edit Response HTML" experiment is one of our most powerful yet, as it allows you to freely edit the initial HTML of a website and compare the performance of that edited version to the live website.

anchorWhat's it for?

The use cases for this experiment are infinite...

  • perhaps you'd like to test the performance impact of reordering some HTML elements in the head of a website to match the recommendations in Harry Roberts' excellent Get Your Head Straight guide...
  • or maybe you'd like to A/B test the addition of new section of your homepage...
  • or you're curious see how much pageweight is saved by swapping out several images with verbose picture element markup.

Each of these situations would have been difficult or impossible to pull off with our existing Find & Replace experiment, yet they are now trivial using this new freefrom editing experiment.

anchorA demo

Your edits can be as simple or dramatic as you need. To demonstrate the power of this feature, here's an experiment I ran that replaced the entire HTML of my personal website with the HTML from Google Flights' homepage instead:

screenshot of example comparison of swapped HTML

See these experiment Results here.

Now, that particular experiment wasn't useful for much other than showing me that from a performance perspective I am probably better off being myself, but you get the idea: this is a new level of control in performance experimentation!

anchorHow to use it

To try it out, you'll need a WebPageTest Pro account... then:

  1. scroll down to the "Create" section of the Opportunities & Experiments page on any test that has the "Save Response Bodies" setting checked at the start (you can find that setting in the Advanced panel).
  2. You'll find the "Edit Response HTML" experiment there, pre-populated with the HTML from the site being tested.
  3. Check the box and make edits, then run your experiment.
screenshot of the form where the experiment is offered

Like all WebPageTest Experiments, the experiment changes are applied to the live website mid-response using our proxy server, and the browser will receive the modified version of the website as if it came from the server. Also, all experiments include a control run so that both the experiment and control are proxied in the same environment.

anchorNo Pro account? Test it out free...

If you don't yet have a WebPageTest Pro account, you can still try out all of the experiments we offer by running a test on our special, poor-performing test site, The Metric Times! You'll just need to sign into your free WebPageTest account first and then you can try running experiments on this test, for example!

Also, we should mention that a couple of our experiments are free to use on any site with your free WebPageTest account. Namely, the "Defer JavaScript" experiment, and the "Add HTML to the end of Head" experiments... both quite useful in their own right!


  • You'll need a WebPageTest Pro account to use this experiment
  • You need to check "Save response bodies" in the Advanced tab when starting a new test
  • "Save response bodies" is not working in Firefox at the moment


This experiment is automatically available today for all WebPageTest Pro users. We hope you love it. As this is a first release of the new feature we appreciate any comments, use cases, bug reports, and feedback.

Scott Jehl is a Senior Experience Engineer at Catchpoint who cares about creating fast, compelling digital experiences that can be delivered to the broadest possible audience. He is the creator of the Lightning-Fast Web Performance Course, author of Responsible Responsive Design (A Book Apart, 2014), and co-author of Designing with Progressive Enhancement (New Riders, 2010). He is also a frequent speaker at web conferences around the world. More at

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