Clarify that the style attribute shouldn't be used in MathML mark up unless otherwise specified

[samimaattaCelia]

Point being that the supplier should avoid using only visually rendered accents and such. (So, not as strict as I made it in the issue heading.)

For example vectors can be represented with a line on top of the variable: $\overline{\mathrm{v}}$ and temml.org's converter outputs this kind of mark up with the style attribute:

<math>
  <mrow style="padding:0.1em 0 0 0;border-top:0.065em solid;">
    <mrow>
      <mi mathvariant="normal">v</mi>
    </mrow>
  </mrow>
</math>

According to Neil (because I asked about this) the correct mark up would be to use the character ¯ and <mover> element:

<math>
  <mover>
    <mi>z</mi>
    <mo accent='false'>¯</mo>
  </mover>
</math>

In this you should also specify that the it's not an accent with the accent='false' attribute.

It didn't come to my mind just yet which other kinds of similar notations could be possible to mark up wrong, so this is something to keep in mind at least. I guess the point being that the supplier shouldn't use the style attribute in MathML mark up when possible.

[samimaattaCelia]

Not to say the style attribute wouldn't be useful for other use cases, but when it comes to accents and things like this, it shouldn't be used.