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>