save with a bookmark keyword (Firefox) or search engine shortcut (Chrome), e.g. "readw" and then run with "readw 30"
javascript:(function() { var width = %s || 40; var styles=`div, p { line-height: 135% } body { margin: 0 auto 0 auto !important; max-width: ${width}em }`; var newSS=document.createElement('link'); newSS.rel='stylesheet'; newSS.href='data:text/css,' + escape(styles); document.getElementsByTagName("head")[0].appendChild(newSS); })();