Yes you need to enabled the HTML5 flag in the application. After that all the harness need to be updated with the document mode to IE=Edge in the Advanced tab. This will allow Pega 7 applicaiton to run on Edge mode even though the compatibility setting is there in browser level.
Please please please consider and advocate to your business/IT to consider Enterprise mode in IE11. It was introduced to specifically address catch 22 scenarios where your IT needs to support legacy applications along with modern applications in parallel.
This way you set Enterprise mode for your legacy app, and then leave IE11 alone, unmodified and untampered with for your modern [HTML5] apps such as Pega 7.
If this is off the table, only then, I would consider local changes from Pega's side. Using this hierarchy of approaches will net you less headache and an overall better user experience.
Thanks for your response, No one in Pega suggesting the local changes. People are refusing to support by stating that we don't support. We are in Catch 22 situation. kindly help us share some of the locale changes as apart changing Harness there are many other places where we would need to apply.