To create an HTML link to a UNC path, use the following format for the href attribute of the anchor (a) tag.

<!--
To create an HTML link to a UNC path, use the following format for the 
href attribute of the anchor (a) tag.

    file://///path/to/unc/share

You must have *five slashes* at the beginning! The first three slashes
are a part of the protocol "file:///" (yes, indeed it is supposed to have
three ... unlike the two in http://, https://, ftp://, etc.). The next two are
the first part of the unc path switched from back-slashes to slashes.

HUGE CAVEAT!!!

  This will only work by default in Internet Explorer and only if the user is on
  the same network as the UNC path. Meaning, this will not work for a publicly
  accessible site. This will only work for an internal intranet-ish site. Only
  IE has the file protocol turned on by default. You can get extensions for
  Firefox and Chrome that will enable it.

Chrome Extension
https://chrome.google.com/webstore/detail/locallinks/fkkidcgelfcjmebocoaicnelkbnpgpmm

Firefox Extension
https://addons.mozilla.org/en-US/firefox/addon/noscript/?src=search
-->

<!-- UNC Link -->
<a href="file://///computer/share/file.mp4">Watch.mp4</a>

<!-- UNC Link w/ spaces encoded (%20) -->
<a href="file://///computer/share/file%20with%20spaces.mp4">Watch.mp4</a>