Yes Apoorva. If you are moving to Pega 7 and using the new technology that comes with it, you will need to be in standards mode and enable the HTML5 doc type.
When you come over from previous releases you may find some functionality does not work when running in standards mode. This is almost always because some of the features you brought over were originally designed with Quirks mode in mind and this will conflict with the HTML5 doc type. Often there are auto-generated, HTML5 compliant replacements for said features and you can upgrade them.
The first place I would explore is the HTML5 readiness wizard. Consider it analogous to a Pega guardrail report, scoped specifically to get you standardized for HTML5.
Then during your upgrade effort if you have any specific issues or functionalities you are having difficulties with, I would recommend you be as specific and detailed as possible when opening an support request or engaging the support community depending on your needs and the context of the issue.