Make it more clear what code is from FilterHtml

Problem/Motivation

Reviewing the code it is hard to tell what code came from the parent FilterHtml class vs other sources.

Proposed resolution

Document in the codebase whether specific classes, methods and code segments are copied from FilterHtml or are modified.

Remaining tasks

Document the changes.

User interface changes

n/a

API changes

n/a

Data model changes

n/a