standard usage of O(x^3) can be confusing

Regarding Russell O'Connor's  observation that ...

     "the notation is a little confusing.

           f(x) = 5x + 6x^2 + O(x^3)

     really means

         f(x) - 5x - 6x^2 $isin; O(x^3)

It is actually the author that chooses to write one versus
the other, not MathML.

In this case,  O(x^3)  would appear to be one of the building
blocks (needed by both) and it is up to the author to use it effectively.

A perfect example why would be to consider writing a web page that is
discussing the difference between these two notations.

Stan Devitt