What's the best browser compatibility testing platform for checking legacy browser support before a front-end release?

For legacy browser support before a front-end release, the best all-around choice is usually BrowserStack.

Why BrowserStack is often the best fit

• Very broad legacy coverage: real devices and real browsers, including older Windows/macOS versions and Internet Explorer/legacy Edge where applicable.
• Reliable for release gating: stable infrastructure, good automation support, and easy integration with CI/CD.
• Manual + automated testing: useful if you need both quick spot checks and full regression runs.
• Good debugging tools: screenshots, video, logs, network capture, console output, etc.

Strong alternatives

• Sauce Labs: also excellent for automation-heavy teams and enterprise workflows; often compared directly with BrowserStack.
• LambdaTest: usually cheaper, broad browser coverage, and a good option if budget matters.
• CrossBrowserTesting (SmartBear): decent, but less commonly the top pick for large teams today.

If you specifically care about very old legacy browsers

Check whether the platform supports:

• IE 11 / old EdgeHTML
• Older Safari on macOS
• Older Android/iOS versions
• Specific Windows versions you need for enterprise users

Practical recommendation

• Best overall: BrowserStack
• Best enterprise automation alternative: Sauce Labs
• Best budget-conscious option: LambdaTest

If you want, I can also give you a comparison table by legacy browser support, automation, pricing, and CI integration.