Since it is your style sheet you are responsible for correct function of the minimized code too. In few cases minimized code may behave unexpected.
Step 1 Select input by (optional) choosing the encoding and a style sheet (.css) file. Charset selection is only needed if the file contains non-ASCII characters (UTF8 is a superset of ASCII). Windows users may find the default charset by opening the PowerShell, input [System.Text.Encoding]::Default and look for the value of WebName. Alternatively encode non-ASCII characters like e.g. \0020AC for the euro sign. Style definitions can also be pasted manually from clipboard (click into text area, then [Ctrl]+[V]).
Step 2 Start the minimizer. Please allow some seconds of delay. The result will appear in the text area below.
Step 3 Copy the content of the text area into clipboard for your use. This can be also done manually by clicking into the text area, then press [Ctrl]+[A], then [Ctrl]+[C].
Step 4 Addendum: Using the apache webserver the following code, in a .htaccess file, is one way to automatically serve [stylesheet].min.js if available and falls back to serve [stylesheet].js if not (tested with apache 2.4):