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Control Panel > Primary > Minify HTML Page Output

Minify HTML Page Output
Remove all unnecessary characters from page HTML without changing its functionality. The process removes unnecessary characters which usually include white space characters, new line characters, and comments, which are used to add readability to the code but are not required for it to execute. This process is done in real time.

This feature was added in UBB.threads 7.6.1 - 2018-01-01
https://www.ubbcentral.com/changelog.php#761

The optional feature is disabled by default.