SSI works fine for many purposes. It's fast, and relatively powerful. However, to write a page with SSI, one must either work with one template or work with a set of rather simple ones. With SSTP, the template is just that: a template for parsing. Any SSTP template will work with virtually any webpage. A set of pages can be retrofitted with an SSTP template in less than a minute. Any set of SSTP pages will work with or without the parser, with all the links and images in tact. With SSI, your template defines the pages. With SSTP, the template and pages are completely independent entities.

SSTP supports all the features of SSI in its embedded tags (although the syntax is somewhat similar, the directives are different.) Unlike SSI, SSTP allows variables to be used in any function, even an include. Also unlike SSI, SSTP reads three levels of variables: those in site.thtml at the site root, those in folder.thtml in the page directory, and those in the page itself. Unlike standard SSI, SSTP supports conditionals and setting or reading any variable, although Apache supports this as well. SSTP also allows you to embed only what is in one tag of a page, and automatically makes things that work as links on disks work as links when parsed. SSTP supports functions to format specific parts of a page according to a template, without the overhead of loading more includes.

With SSI, you design a page for SSI. With SSTP, you design a page for HTML/XHTML. A properly designed SSTP template does not require any special page tags at all.