In addition to the attributes described specifically for each element, the attributes: id, xref, class, style and href are allowed on every MathML element. MathML Fundamentals
<math display="block"> <mspace linebreak="newline"></mspace> <mrow style="border:1px;"> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mo style="border:2px">→</mo> <mo>+</mo> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mi>very long</mi> <mo>+</mo> <mo style="border:2px">→</mo> <mo>+</mo> <mi>very long</mi> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mi>very long</mi> <mo>+</mo> <mo style="border:2px">→</mo> </mrow> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px;width:600px"> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mo style="border:2px">→</mo> <mo>+</mo> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mi>very long</mi> <mo>+</mo> <mo style="border:2px">→</mo> <mo>+</mo> <mi>very long</mi> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mi>very long</mi> <mo>+</mo> <mo style="border:2px">→</mo> </mrow> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px;width:800px"> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mo style="border:2px">→</mo> <mo>+</mo> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mi>very long</mi> <mo>+</mo> <mo style="border:2px">→</mo> <mo>+</mo> <mi>very long</mi> </mrow> <mspace linebreak="newline"></mspace> <mrow style="border:1px"> <mi>very long</mi> <mo>+</mo> <mo style="border:2px">→</mo> </mrow> </mrow> </math>