Dandelions are often a favorite for rabbits! Just ensure that that they have not been sprayed with anything. Also, if it is new to your rabbit, go slowly (small amounts) with introductions just as you would for any new type of green.
Domestic rabbits have lost their ability to know what plants are poisonous. They may certainly eat a toxic plant as easily as they would chow down on a candy bar.
Dandelions are our bunnies' favorite food this time of year. I know of a couple of patches where no herbicides are sprayed and the flowers are huge. Best of all, they grow thick and tall; so, harvesting them is easy.
Dandelions fill up almost half of my buns' vegetable consumption from roughly may to october, the other almost half being "ground elder" plant. We have them in plenty and more than a pair could consume, i've even dried a bunch of these along with other garden greens into a treat hay type thing.
It's worth remembering that while they're excellent food,if not contaminated,theey are quite high in calcium and are a diuretic,so very large amounts could lead to 'stones' and excessive urination.Moderation in all things,as has been pointed out.