Описание тега shtml
NoneShtml files are HTML files that undergo server-side includes processing
SHTML files are HTML files for which server-side includes (SSI) is enabled.
Usually the server knows that it must perform SSI processing because the filename ends with a recognized extension, typically .shtml, .stm or .shtm. If the server is configured to allow it, SSI-enabled files may be recognized because they have the execution permission bit set.