Noob here. In my gmail $\triangle ABC \cong \triangle XYZ$ gives error [\cong?] while replacing \cong with \sim, \neq, \approx, or \simeq, etc all render just fine....adam-p commented Aug 29, 2014 For TeX rendering, you're at the mercy...Markdown Here options page, in the "TeX Mathematical Formulae Support"...