China’s CSI300 and Shanghai Composite rallied on reopen as investors cheered the U.S. tariff ruling’s potential export relief.Summary:CSI300 +1.4%, Shanghai Composite +1.2% at openRally follows nine-day mainland holidayInvestors cite U.S. Supreme Court tariff ruling as positive for exportsHang Seng reverses after Monday’s 2.5% surgeAnalysts see potential easing of U.S.–China tariff pressureChinese equities opened sharply higher on Tuesday as traders returned from a nine-day holiday break, buoyed by optimism that recent developments in U.S. trade policy could ease pressure on Chinese exports.The blue-chip CSI300 Index rose 1.4% at the open, while the Shanghai Composite Index gained 1.2%, with buying broad-based across sectors including consumer electronics and machinery.Market participants pointed to the U.S. Supreme Court’s decision to annul President Donald Trump’s “reciprocal” emergency tariffs as a catalyst for improved trade sentiment. Although Trump subsequently announced a temporary 15% global tariff, analysts say the reset may ultimately translate into relatively lower effective tariff rates for China compared with previous proposals.The legal ruling has injected fresh uncertainty into global trade policy. U.S. equity markets fell on Monday amid confusion over the future tariff framework. However, investors in mainland China appeared to interpret the development as potentially constructive for Beijing, particularly if it weakens Washington’s leverage in future trade negotiations.In Hong Kong, the mood was more cautious. The Hang Seng Index slipped more than 1% after rallying 2.5% on Monday in reaction to the tariff headlines. The pullback suggests some profit-taking following the prior session’s surge.The rebound in mainland shares also reflects pent-up positioning flows after the extended market closure. With global investors reassessing the implications of the U.S. court ruling, Chinese stocks may benefit from renewed expectations that export headwinds could moderate in coming months.Still, trade policy uncertainty remains elevated. While the Supreme Court decision curtailed certain emergency powers, the administration retains other legal avenues for tariffs, including national security provisions.For now, traders appear focused on the prospect that the latest shift in Washington could provide incremental relief to Chinese exporters and improve the tone of bilateral negotiations.China's best friend! This article was written by Eamonn Sheridan at investinglive.com.