What are the various opinions regarding wearing crocs on Tisha B’Av.
The principle prohibition, as mentioned by poskim, is to wear leather shoes on Tisha Be’Av. Crocs are synthetic and do not contain any leather.
Yet, poskim dispute (concerning Yom Kippur) whether or not one may wear non-leather shoes that are comfortable. This dispute begins with rishonim (Rambam/Rashi) and continues with acharonim (see Shulchan Aruch 614:2; Mishna Berurah 5), and although many poskim write that one should be stringent and avoid comfortable shoes, the general custom is to be lenient.
On one hand, Tisha Be’Av is not as stringent as Yom Kippur, which is grounds for leniency. On the other, some poskim mention that on Yom Kippur it is wrong to be stringent, because there is no reason to deprive oneself of comfort beyond the five inuyim, whereas on Tisha Be’Av it is right to be stringent. This is an involved topic, and please G-d we will dedicate an article to it in the future.
Concerning crocs, the pamphlet “Halacha U’Maaseh” asked the leading poskim their opinion, and many commented that although permitted, it is better not to wear crocs (because of their being comfortable). Yet, they are no worse than comfortable sneakers, and similar footware that many use.
[Rav Elyashiv is quoted are prohibiting crocs, but it is hard to know how the question was presented.]