SVG and PNG math renders in the MathML render mode are pulled in as background images so that browsers with MathML support do not load SVG fallback images. We now know that it is most likely not required to have the images be background images and that as background images there are two problems: browsers default to printing without background images so the math is not printed and math can not be copied and pasted. The following solutions have been suggested: convert images to <img> or inline SVG (no known problems) or a couple of workarounds described below that may nor may not work for all browsers and the effects on different setups are currently unknown.
However, browsers default to printing without background images, which means that formulas go missing when printing web pages using default settings. Users can work around this by manually checking "print background images" in the print settings, but it is likely that they won't notice the missing formulas before holding a print-out in hand.
There is a CSS workaround for Webkit, which can be targeted at math elements: -WebKit-print-color-adjust: exact;. This makes Webkit print background images in the element this style is applied to, regardless of print settings.
However, it is not clear if there is a solution that will work reliably for other browsers. http://maxlapides.com/forcing-browsers-print-backgrounds/ discusses several options, but we'll have to test whether any of them can be made to work reliably for math. Formulas seem to be missing in print-outs even when using Firefox with the MathML plug-in.
If no workaround can be found, then this brings us back to the perennial question of whether MathML is worth the hassle. If we dropped MathML for regular SVG images, then this problem would go away.
This change means that it is no longer possible to copy and paste math (from Chrome at least). It might help to make the SVG fallback an actual inline image, but I can't even seem to get that to copy and paste.
I know a lot of people that copy and paste equations from Wikimedia sites (WP and WB) as a quick and easy way of displaying them in presentations and the like, and this is going to break that.