MathJax supports manual numbering using $$\tag{…}$$, as in \eqref{grz} below:

$$\tag{1}\label{grz}\Box(\Box(p\to\Box p)\to p)\to p$$

Cross-references work, too, with \label and \eqref. The tags needn't be numbers:

$$\tag{LR}\Box p\to p\vdash p$$

MathJax supports manual numbering using $$\tag{…}$$, as below:

$$\tag{1}\label{grz}\Box(\Box(p\to\Box p)\to p)\to p$$

$$\tag{1}\label{grz}\Box(\Box(p\to\Box p)\to p)\to p$$

Cross-references using \label and \eqref work as well: \eqref{grz} \eqref{grz}. The tags needn't be numbers:

$$\tag{LR}\Box p\to p\vdash p$$

$$\tag{LR}\Box p\to p\vdash p$$