The capital gains tax has been raised three times since 1969. The first two times were in isolation and were followed by market selloffs. The last tax increase was part of comprehensive tax reform in 1986 which slashed tax rates on income and corporate taxes. The S&P initially sold off on the inclusion of the higher capital gains tax rates, but the 28% income and 33% corporate tax rate overwhelmed the negative impact of higher cap gains.
Of course, this time around the rise in investment taxes will be part of a broader tax-hike package, which does not bode well for the market.