There are many plugins that do minification available for free. A sample of those known to work well when using WordPress as a static site generator are:
Depending on your active theme and other changes unique to your WordPress site, you may need to play with adjusting the settings with these plugins. If the resulting site is not rendering correctly, try turning off all minification settings and then turning them back on one by one until you have as much minification as possible, whilst keeping your site rendering perfectly.
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