The help for displaying a formula suggests to use PNG images in order to write \oiint and \oiiint:
https://en.wikipedia.org/wiki/Help:Displaying_a_formula#.5Coiint_and_.5Coiiint_as_PNG_images
https://en.wikipedia.org/wiki/Help:Displaying_a_formula#Oriented_.5Coiint_and_.5Coiiint_as_PNG_images
I actually first noticed that on https://en.wikipedia.org/wiki/Gauss%27s_law#Integral_form
This results in poor rendering even in the MathML mode. This is also not accessible, or more precisely it splits the MathML formula into several parts (MathML and HTML <img>) which can confuse assistive technologies.
The MathML operator dictionary contains the following characters for that purpose (http://www.w3.org/TR/MathML3/appendixc.html):
∮ ∮ contour integral
∯ ∯ surface integral
∰ ∰ volume integral
∲ ∲ clockwise contour integral
∳ ∳ anticlockwise contour integral