Opening this page runs the real application, invisibly, and puts it through every check below. Typing messages, pasting things into the wrong box, going over the size limit. Green means it did the right thing just now, on your computer, in your own browser.
You do not have to understand every line. If the bar below is green, the tool behaved. If it is ever red, something is broken and the application should not be trusted until it is fixed.
Back to the appFor developers: this page loads index.html in a hidden frame and asserts against its live DOM, so a regression in the shipped file fails here rather than in a copy of the logic. It has to be served over http, because a browser will not let a page opened from disk read into its own frame.