Recently we had to set up a new AB testing and personalization tool (client-side).
We started with a simple setup by loading the tool in the tag manager. Our tag manager is loaded async so the tool was also executed async which means the script is executed while parsing the page.

So the original content was shown and then replaced by the personalized content and causes the website to flicker.
It is understandable this happens because of the delay the tool script execution has to endure.

Most A/B testing tool vendors provide a “solution” for this problem called a “Pre hiding snippet” or “Anti-flicker snippet”. Here are examples from Adobe and Google.
Yes, it will stop your website from flickering, but the solution itself makes me very unhappy. What it does is turning your website completely white for the duration it takes to load the A/B testing tool.

This is not a “solution” but a “workaround” for an ugly side effect of slow execution of the A/B testing tool.

What I did was set up a number of tests where I load the AB testing tool in different ways and also remove the initiation from the tag manager to the website template/code (back to the old ways…).

Now the winner is: Load your AB testing tool as soon as possible on the page and sync. Now the flickering was gone in every browser except IE11 (which is a crappy browser anyway and is end-of-life).