From
http://www.math.union.edu/~dpvc/jsMath/authors/spriteImageFonts.html

The spriteImageFonts plugin

    This is an experimental plugin designed to address some of the
    important drawbacks to the traditional implementation of the
    image-font fallback method in jsMath. Currently, each character in
    each size in each font is a separate image, so there are tens of
    thousands of tiny images. That means that there is a lot of
    overhead both in terms of network traffic when the image fonts are
    used and in storage space on the server to make image fonts
    available. For those who must use FTP to move individual files to
    the server, it can be quite painful to install the image fonts.

    The spriteImageFonts plugin uses an alternative approach in which
    one larger file stores the images of all the characters in a given
    font, and only a small portion of that file is displayed when one
    of its characters is needed. This has the advantage of reducing
    the number of needed files by a factor of 128 (the number of
    characters in each font), so they take up far less space on the
    server, and there is less network activity needed when a user
    requests image fonts.

    To use this plugin, you need to download the sprite font images
    (available on the jsMath download page) and put them in the jsMath
    directory on your server. They are stored in a directory called
    fonts-sprite, and can be used without replacing the existing fonts
    directory, if desired. If you always use the spriteImageFonts, you
    can remove (or not download in the first place) the fonts
    directory and just use the fonts-sprite directory. If you are
    using any of the extra fonts, you should download sprite-based
    versions of those fonts as well and place them in the fonts-sprite
    directory.

    Then, on those pages where you want to use the sprite-based image
    fonts, add the command:

    <SCRIPT SRC="path-to-jsMath/plugins/spriteImageFonts.js"></SCRIPT>

    before loading jsMath.js or the autoload plugin, but after setting
    the jsMath variable (if you are customizing jsMath in other
    ways). This will cause jsMath's image mode to use the new
    sprite-based image fonts rather than the traditional image fonts.

    Note that this is considered an experimental plugin, and so you
    should use it with caution. The image-fonts are the main fallback
    method for when the TeX fonts are not available, so you need to be
    sure the benefits are worth the costs that are outlined in the
    next section. Also, there is no guarantee that this plugin will be
    maintained in later versions of jsMath. (If it turns out to be too
    problematic, it may be dropped.)

Known Problems with the spriteImageFonts plugin

    The main reason to use sprite-based images is that they should
    transfer to the user's browser faster (they should generate less
    network traffic), and because they take up less room and fewer
    files on the server.

    Unfortunately, these benefits come at a cost. One problem is that
    the clipping required to obtain the small images from within the
    larger file is very delicate, and even a one-pixel error can be
    disastrous, especially for fonts at smaller point sizes. While the
    on-screen rendering works reasonably well, some browsers introduce
    slight spacing errors in the printed versions of their pages, most
    likely due to the changes in font metrics between the screen and
    printer versions of the fonts. These errors can lead to slight
    clipping of characters in the printed output.

    A more serious problem is that some browsers render the clipped
    images slower than they do the individual images (presumably they
    are rendering the whole image even though most of it is not
    shown). This is particularly noticable in MSIE on the PC, which is
    already the slowest browser that handles jsMath, making it even
    slower. Indeed, the images that use alpha-channel transparency are
    so slow in MSIE on the PC (due to the rediculous apppraoch used to
    handle alpha-channel transparency in MSIE) that the non-alpha
    images are used by default (in MSIE only) unless the user
    specifically requests alpha transparency.

    Speed also seems to be an issue with Firefox on the PC when the
    high-resolution images for printing are used. This causes Firefox
    to run unexpectedly slowly, though it does not seem to affect the
    normal screen resolution images. In addition, Firefox's print
    preview on the PC will show the images located in the wrong
    positions, but they will print properly anyway.

    Opera on both the PC and the Mac has significant problems with the
    sprite-based image fonts. Due to a serious alignment bug in Opera,
    the sprite images can't be handled as standard images, so are
    implemented using background images instead. This means they can't
    be scaled by jsMath, and so the scaling and hi-res printing
    options are disabled in Opera. Since Opera automatically scales
    images when you zoom the page, this is not really much of a
    problem, though it does mean you can't get high-resolution
    printing. Since background images are not printed by default,
    however, in order for Opera users to print pages containing
    mathematics, they need to turn on printing of background images in
    the print dialog box. This works fine on the Mac, but on the PC,
    the images don't print properly, even though they show up
    correctly in the print preview. (It looks like the positioning of
    the background image is not being adjusted correctly for the
    resolution of the printer, and so the font image is positioned
    incorrectly and the wrong characters are printed.)

    Finally, some older browsers do not implement the clipping that is
    required to make the sprite-based fonts work, and so the image
    modes are disabled for these. That means that if the user does not
    have the TeX fonts installed, the only option will be unicode
    fonts. These are not as effective as the traditional image fonts,
    so changing to sprite-based fonts will cause a reduciton in
    quality for users of these older browsers.

    All of these problems make the sprite-based image fonts less
    reliable than the individual-image fonts, and so the sprite images
    have not been incorporated into jsMath.js directly. You may choose
    to use the sprite fonts if the reduction in storage space or
    number of files is critical enough to compensate for the problems
    that using them introduces, but it is a decision that should not
    be taken lightly, as it will inconvenience some of your users,
    perhaps significantly.
