Running non-compliant extensions is your right, but the results don't need to be supported. I've run into this situation a number of times, although not with adblock. My advice has been to disable the extensions, or use a different browser without them.
You're leaving out the option of "not using the website."
Let's face it, nothing for sale on TeeSpring is essential. Just thank them for putting a roadblock on your silly impulse buy.
Compliant to who/what exactly ? Extensions are executed in a user agent, ie a software acting on behalf of a user based on the settings they prefer. If anything it's the website that's non-compliant with the user's choice.
It's your choice to use things that don't conform, such as IE11, but I wouldn't assume it's the server's problem.