S&P 500 KEY POINTS:The S&P 500 has risen more than 20% in 2021 without suffering a single drop of at least 5%Slowing growth, deteriorating market breadth, high valuations and seasonal weakness pose headwinds heading into the fallHowever, the prospect of an increase in corporate taxes may be the most important risk to consider for the equity market in the short term.The S&P 500 has soared since the end of March 2020 and is up more than 20% in 2021 alone. Last week, the index notched its 55th record closing high of the year, after rising for the seventh consecutive month in August. The non-stop rally, which has gone all year without a drop of at least 5%, has been fueled by unprecedented fiscal and monetary support in response to the coronavirus pandemic. Retail investors, who ha