Repro:
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Get a bootstrap css. I got mine from bootswatch
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It's got selectors with hex values like this:
.glyphicon-download:before {content: "\e026";}
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Run the file through the text minifier.
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Then find that selector, and find that the value of content is replaced by a square.

(Notepad)

(Notepad++)
Obviously the minifying is occuring in Microsoft.Ajax.Utilities.Minifier.MinifyStyleSheet(), and nothing to do with this application, but worth noting.
Is this an issue with the minifier or with the display of a char?
If so, how can we report it out?
The NuGet package is referencing Assembly AjaxMin.dll, v5.9.5229.26434