8  Boxes, Dimensions, and fills

Boxes, dimensions, and fills are rarely appropriate for web documents because they imply an attempt to control the fine details of layout. Browsers make their own choices about layout of a document in HTML. For example they make the lines fit whatever size of window happens to be present. This dynamic formatting makes mincemeat of most detailed TEX layout. In fact, if you want your readers to see exactly what you see, that is impossible with HTML, and you should use some other representation of your document.
There are nevertheless many cases when a TEX document containing boxes, dimensions, and fills needs to be translated. Limited translation of these constructs is supported. They are translated, where appropriate and possible, into HTML tables with widths and vertical skips estimated to give a reasonable result on a browser. It must be stressed that accurate translation is inherently impossible because browsers deal in pixel sizes and default font sizes that vary and are out of the control of the publisher.
The types of box usage that translate quite well are when things like
\hbox to \hsize{The left \hfil the Right}
\vbox{\hsize=2in Matter to be set in horizontal mode to a 
  limited hsize}
\makebox[0.6\hsize][r]{Stuff to the right of the makebox.}

are on a line by themselves. You get:
The left the Right

Matter to be set in horizontal mode to a limited hsize
Stuff to the right of the makebox.

Usages that translate poorly tend to be boxes within a line of text. That is because current HTML table implementations have to start a new line unless they happen to be adjacent to a table already. Thus an hbox in a line will give a line break that you might not have wanted. This behaviour is really a bug in the browsers, but we are currently stuck with it. The behaviour of HTML tables is buggy [see 13.5] when their alignment is specified, which means that strange results are likely if more than one box on a line is being set. Boxes in equations are troublesome. The only type that is reasonably supported is \mbox which is often used in LATEX for introducing text inside equations.
Negative skips are not supported at all.
The only important dimension parameter that is currently interpreted is \hsize. It is what controls the width of a vbox. It can be reset using the plain TEX format e.g. \hsize = 3in or scaled or advanced e.g. \hsize=0.6\hsize but only within a group. It makes no sense for the HTML file to try to specify the width of the line at the outermost level. That is the browser's business.
New dimensions can be defined, set, advanced, scaled and used to set other dimensions including \hsize.
TTH trys valiantly to mimic the sort of text alignment that is obtained using glue such as \hfil and \hss, provided it is inside a box. However, the alignment algorithm of HTML tables makes it impossible to obtain fills with exactly equal sizes. So don't be surprised if some results looks disagreeable. Moreover, TTH will completely ignore the glue outside an hbox, and it doesn't know the difference between \hfil and \hfill, etc.