Just decided to compile one, might be useful if somebody is actually ever going to fix them:
1. "-fonts X" doesn't do anything at all. CSS can't see it.
2. Using the character '@' anywhere in the stylesheet causes the game to use the default styleheet instead (this is really bad, it prevent one from using media queries, keyframes and custom fonts, for example).
3. The website states b, i, u and q HTML tags can be used in descriptions, yet only b and i are processed.
4. The top bar displays neither the game author nor game title.
5. "-unlocks with X" doesn't do anything
6. "X1>X2" always returns X1 instead of a random number between X1 and X2.
6. If the amount of the resource reaches 0 and stays there for a second it stops being displayed, regardless of visibility settings, unless the amount starts at 0.
7. Numbers that should be zero are sometimes displayed as extremely small (1e-138).
8. "multiplies income of X by Y%" doesn't do anything (keep in mind it's only when X is a resource).
9. -visible at 1 X, 1 Y produces some really weird behavior (check this
10. Represents and occupies only work when the building is manually bought and not when it is obtained otherwise (though it allows for in-game buy 10 buttons, so this doesn't have to be fixed ;) ).
11. "requiring X Y" doesn't work with "converts A to B", even though it's described as working with any "power" (if anyhting, the description could be more specific). In this case converts will always execute regardless of the amount of Y.
12. -"gives X Y when bought" freezes the game until it is reseted.
Those are just the 12 ones I've found. Fell free to add the ones you've encountered, so if Orteil gets to updating IGM somewhere in 2018 he will know where to starts from.