Unfortunately, when Joomla implemented Bootstrap, they didn't do so completely, and so we have some incompatibilities in places. One such incompatibility is between the bootstrap (Glyphicons) icons and the Joomla (Icomoon) icons.
So for example (and this is one of many):
Bootstrap has two plus sign icons: icon-plus (a plain +) and icon-plus-sign (a + in a circle)
Joomla has two plus sign icons: icon-plus (a + in a circle) and icon-plus-2 (a plain +)
In T3, if you use the icon-plus-2 icon in e.g. a button, it displays empty!! But using protostar it shows an icon.
Aaaaaaarrrrrrrrgggggggghhhhhhhhhhh!!!!!!!!!!! Why, why, why, WHY, WHY did the Joomla team do this??????
More to the point, How can we improve the compatibility of T3 and Joomla?
There are 130 icons in Joomla. There are 140 icons in Bootstrap. T3 uses FontAwesome which is a superset of Bootstrap and provides 378 icons (some of which are aliases). Of the 130 icons in Joomla, 42 have the same name as a FontAwesome icon, and the remaining 88 don't map - but many of these have identical or similar-looking icons which we can map onto.
The css for icons looks like it can only map to one icon set - so the obvious choice is to stick with the FontAwesome icon set and try to provide greater compatibility.
1. At a minimum we should map the 88 Joomla icons to the closest FontAwesome icon. This should be as easy as adding the names to icons.less - I will do this and create a PR for it. This will at least avoid empty icons and be semantically close.
2. However, ideally we would also provide an admin selectable option whereby we also remap the names which are common to both Joomla & Bootstrap names so that the icons shown are the closest looking FontAwesome icon to the standard Joomla3 one. The user can then choose whether they have full compatibility with other Bootstrap templates / web pages or with other Joomla templates (like protostar).
I am not sure how much effort it would be to do item 2.
S