For MSHtml engine
This property determines or reflects the virtual browser width when
HTML documents do not have a predefined width and height. The
width of an HTML document varies as the client resizes the browser.
How far the content flows down the page is dependent on the width
of the browser.
The Width parameter is used to control this aspect of HTML
rendering. It is exactly as if you were displaying your HTML in a
browser window the same width as the supplied value. Typical values
might be 640 or 800.
If the Width is zero then the web page view will default to a
size which shows all the available content without needing to
scroll from left to right. You can use this default to ensure that
all your HTML content is visible.
For Gecko engine
This value determines the paper width, in pixels,
to use while doing layout in Gecko. Since the Gecko engine always
render in a paginated context and never needs to scroll, contents
that horizontally exceed this value will often be reflown vertically. For example, a
sidebar done using the CSS float property may appear below
the "main content area" because the paper is not wide enough to
accommodate both items. If the value is 0 the paper width will be
determined from the current Rect's dimensions.