Shared Borders are an old technology used by FrontPage (1995-2003, RIP) to enable consistent page layouts; the borders also enabled uniform boilerplate text (header, footer etc) and links (navigation) surrounding unique content on each page. The shared borders were constrained to surround the page content- top, bottom, left and right - but individual borders could be turned on or off as desired.
FrontPage 2003 deprecated shared borders and introduced Dynamic Web Templates (DWT) as a replacement. The DWT can be considered to be a super-shared border, only one is required and the boilerplate can be placed (almost) anywhere in the page. And unique content can be placed in the middle of the boilerplate.
Expression Web, up to version 3, continues to support Shared Borders in so far as pages with them retain the same functionality as in FrontPage. However, Expression Web 4 does not, and can utterly destroy any page with Shared Borders.
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