SVG output processor uses Scalable Vector Graphics to render
the mathematics on the page. SVG is supported in all the major
browsers and most mobile devices; note, however, that Internet
Explorer prior to IE9 does not support SVG (MathJax version 3 doesn’t
support these in any case), and IE9 only does in “IE9 standards mode”,
not its emulation modes for earlier versions. The SVG output mode is
high quality, and displays and prints well in all browsers. Since it
uses SVG data instead of font files, it is not affected by user-based
web-font blocking, or other character placement issues that sometimes
occur with the HTML-based output.
One advantage to the SVG output is that it is relatively self-contained (it does not relay heavily on CSS, though it does use some in certain circumstances), so it can be saved and used as an independent image. One disadvantage of this mode is that its variable-width tables become fixed size once they are typeset, and don’t rescale if the window size changes (for example).
In version 2, equation tags and numbers where produced using a fixed width as well, so the equation number would not change with changes in window size. In version 3, however, equation numbers now are based on the container size, and move with changes in its size, just as they do with CommonHTML output.
Finally, because mathematical characters in SVG output are produced by SVG paths, not characters in a font, they can’t be copy and pasted, as the output of the CommonHTML processor can.
See SVG Output Processor Options for information about the options that control the SVG output.