Firefox users won't see these visual clues!
While trying to replicate this for 5.5.2 RN I came to the conclusion that the icons are rendered incorrectly in Firefox (51.0.1 on Ubuntu-Mate 64bit) - cannot be seen at all, while the code for them IS there; probably rendered away from (we think below) the parent element:
(See attached screenshots)
According to Michal (thx), adding the following definition to the .icon-warning::before class could solve this issue (Same thing with the .icon-error::before class.):