[danadam]

I have a bookmarklet, since forever, labelled "sane width", with the following code:

  javascript:(function(){var newSS, styles='body { width: 800px ! important; margin: auto !important } '; if(document.createStyleSheet) { document.createStyleSheet("javascript:'"+styles+"'"); } else { newSS=document.createElement('link'); newSS.rel='stylesheet'; newSS.href='data:text/css,'+escape(styles); document.getElementsByTagName("head")[0].appendChild(newSS); } })();

It forces the body width to 800px and centers it. Crude but it is enough for me.