dy/dx

[mathnewbie]

For the Leibnitz notation for the derivatice should one use:
<mfrac>
<mrow>
<mo>&DifferentialD;</mo>
<mi>y</mi>
</mrow>
<mrow>
<mo>&DifferentialD;</mo>
<mi>x</mi>
</mrow>
</mfrac>

-OR-

<mfrac>
<mrow>
<mi>dy</mi>
</mrow>
<mrow>
<mi>dx</mi>
</mrow>
</mfrac>

Basically, is &DifferentialD simply the lower case version of Euler's differential operator (and therefore should only be used in those contexts)?

As far as appearance is concerned, this appears to capture textbook appearance most closely:
<mfrac>
<mrow>
<mi mathvariant="italic">dy</mi>
</mrow>
<mrow>
<mi mathvariant="italic">dx</mi>
</mrow>
</mfrac>

Are there any plans for Firemath to have a palette of composite elements like dy/dx?