Note that with fractional pixel positioning, the space between two
    given letters isn't fixed, but determined by the accumulation of
    previous rounding errors in glyph positioning.
    
    
      6. WYSIWYG text layouts
    
    As you probably know, the acronym WYSIWYG stands for "What You See Is
    What You Get".  Basically, this means that the output of a document on
    the screen should match "perfectly" its printed version.  A
    true WYSIWYG system requires two things:
    
      - 
        device-independent text layout This means that the document's formatting is the same on the
        screen than on any printed output, including line breaks,
        justification, ligatures, fonts, position of inline images, etc. 
- 
        matching display and print character sizes The displayed size of a given character should match its
        dimensions when printed.  For example, a text string which is
        exactly 1 inch tall when printed should also appear 1 inch
        tall on the screen (when using a scale of 100%). 
It is clear that matching sizes cannot be possible if the computer
    has no knowledge of the physical resolutions of the display device(s) it
    is using.  And of course, this is the most common case!  That is not too
    unfortunate, however, because most users really don't care about this
    feature.  Legibility is much more important.
    When the Mac appeared, Apple decided to choose a resolution of
    72 dpi to describe the Macintosh screen to the font sub-system
    (whatever the monitor used).  This choice was most probably driven by
    the fact that, at this resolution, 1 point equals exactly
    1 pixel.  However, it neglected one crucial fact: As most users
    tend to choose a document character size between 10 and 14 points,
    the resultant displayed text was rather small and not too legible
    without scaling.  Microsoft engineers took notice of this problem and
    chose a resolution of 96 dpi on Windows, which resulted in slightly
    larger, and more legible, displayed characters (for the same printed
    text size).
    These distinct resolutions explain some differences when displaying
    text at the same character size on a Mac and a Windows machine.
    Moreover, it is not unusual to find some TrueType fonts with enhanced
    hinting (technical note: through delta-hinting) for the sizes of 10, 12,
    14 and 16 points at 96 dpi.
    The term device-independent text is, unfortunately, often
    abused.  For example, many word processors, including MS Word, do
    not really use device-independent glyph positioning algorithms when
    laying out text.  Rather, they use the target printer's resolution to
    compute hinted glyph metrics for the layout.  Though it
    guarantees that the printed version is always the "nicest" it can be,
    especially for very low resolution printers (like dot-matrix), it has a
    very sad effect: Changing the printer can have dramatic effects on the
    whole document layout, especially if it makes strong use of
    justification, uses few page breaks, etc.
    Because glyph metrics vary slightly when the resolution changes (due
    to hinting), line breaks can change enormously, when these differences
    accumulate over long runs of text.  Try for example printing a very long
    document (with no page breaks) on a 300 dpi ink-jet printer, then
    the same one on a 3000 dpi laser printer: You will be extremely
    lucky if your final page count didn't change between the prints! Of
    course, we can still call this WYSIWYG, as long as the printer
    resolution is fixed.
    Some applications, like Adobe Acrobat, which targeted
    device-independent placement from the start, do not suffer from this
    problem.  There are two ways to achieve this: either use the scaled and
    unhinted glyph metrics when laying out text both in the rendering and
    printing processes, or simply use whatever metrics you want and store
    them with the text in order to get sure they are printed the same on all
    devices (the latter being probably the best solution, as it also enables
    font substitution without breaking text layouts).
    Just like matching sizes, device-independent placement isn't
    necessarily a feature that most users want.  However, it is pretty clear
    that for any kind of professional document processing work, it
    is a requirement.