This is something I need to have defined before writing a single line of code.
We split the browser-matrix into "Not supported", "Has to work but we won't pixel-push" and "Needs to work and look 💯.
We'll look at any bugs, even from older browsers, though. Fixing them depends on the cost/benefit.
We do large e-commerce-websites, and our clients usually have some old online-shop which is getting replaced. In that case we use their current stats and support whatever makes significant revenue. Usually the client is fine with that.
For my current project, iOS/MacOS-Safari 9-11 make almost half off all sales, with Chrome and FF on the second place. We support the latest Samsung-Internet and Edge/IE11, IE10/9 are quasi non-existent in their stats, so we don't support that.
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