This is what they show instead of a checkmark:

this only happens for some people..it's weird..has to be something with the character set..I'll have to think on a way to fix this..might have to end up using PNGs

In case this is important, I was using Faithful 32x Modded Edition

so..this would be on the resource pack author to add proper char support..I still do plan to change the checkboxes when I get time from characters to pngs..but that'll be a while yet

swapped unicode checkboxes for ascii "x"