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"An mlabeledtr element represents one row in a table that has a label on either the left or right side, as determined by the side attribute. The label is the first child of mlabeledtr, and should be enclosed in an mtd. The rest of the children represent the contents of the row and are treated identically to the children of mtr; consequently all of the children must be mtd elements. "
MathJax relies on this element to implement equation labeling.
Note that this is not implemented in Firefox and that's one of the main reason why MathJax no longer uses Firefox's native MathML by default.
Testcase from the W3C test suite: