Environment-independent method to get the text content of HTML

[RunDevelopment]

Motivation
In #2977, I added a pretty hacky way to get the text content of an HTML string. This was necessary because the method has to work in both browsers and NodeJS and should behave consistently across environments.

Description
Add a method to get the text content of an HTML string. It has to be:

1. Safe: No script execution or left-over tags
2. Correct: No left-over tags or entities
3. Consistent: Same result in all browsers/environments.