Median change to the underlying markup: zero — in every one of six runs, across three samples drawn at different times. That sentence needs about two thousand words of context before it means anything, including the three occasions where I nearly published a number that was wrong. Here they are. Start with what it does not measure: whether a widget helps anyone. It measures whether a page's markup changes when the widget runs. A tool with a genuinely useful toolbar — bigger text, a readin...