> doesn't it make sense ... to be running all kinds of Web browser ... for testing purposes?
No. No, it doesn't.
If you're building a web page, validate its HTML against W3C standards. If it's broken, fix it.
If you're writing a browser, validate its performance against standard HTML. If it doesn't render standard HTML, fix it. If it doesn't render non-standard HTML, who gives a damn?
And even IF you were stupid enough to be running obsolete and vulnerable software for testing purposes, you wouldn't be testing by surfing to dodgy Chinese (allegedly) web sites, or reading your spear phishing email, would you?
Posted from a supposedly secure government network, using IE 6.0. Sigh.