What is the process of ensuring that web pages that are coded with new or advanced techniques are still usable in browsers that do not support the new techniques?

Progressive enhancement

Explanation:

When web applications are built, most times it is necessary to test them on various old and new browsers to ensure that new techniques or features are supported or usable by those browsers. The modality and methodology involved in doing this is called progressive enhancement. With this methodology, web developers focus on building web apps as well as ensure an accessibility balance in the various devices that will access these applications.

Progressive enhancement enforces a rule or logic that allows developers build their web applications from the simplest simplicity to the simplest complexity such that all browsers or other accessing applications render these web applications well.