The S&P 500 is headed toward a 28% advance for 2021 and has hit 70 highs. It is the third straight year of double-digit gains for the broad index, and the second in the midst of the Covid-19 pandemic. The Dow Jones Industrial Average and Nasdaq Composite have gained 19% and 22%, respectively, this year, helping send the major indexes to their best three-year performance since 1999.