With the bookmarklet of Bryce Hanscomb you can test if your website or webapp is usable without any readable text.
With text
Without text
The bookmarklet loads a font which looks like the example above and then overwrites the font-family of all element with this specific font.
At the moment there is a bug that icon fonts will get overwritten too. I’ve fixed this for the icon fonts I use mostly and created a pull request with the bugfix. https://github.com/brycehanscomb/exittext/pull/1
You can find the bookmarklet here: http://brycehanscomb.github.io/exittext/