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  javascript:(function(){document.styleSheets[0].insertRule("* { user-select:text !important }", 1);})();

Extra treat: this other one allows to copy text and open the context menu in pages that are written by rats who disable it:

  javascript:['copy','cut','paste','contextmenu','selectstart'].forEach(e=>document.addEventListener(e,e=>e.stopImmediatePropagation(),true));