Have a HTML-File with embedded javascript.
It will be rendered by a variable replacement process (Arduino) which replaces all occurrences of %variable% by their respective values before delivering.

Execute:

minify manage.html -o ../compressed/manage.html

I get the following error:

ERROR: cannot minify manage.html: unexpected % in expression on line 115 and column 29
4: var maxfilesize = %MAXSIZE%,

The code above is located in the <script>...</script> section.