IE has another issue in regards to testing. A Windows machine will only ever have one version of IE installed at a time: if you’ve updated to Edge, you can’t keep a copy of IE9 on your system. By contrast, installing multiple versions of Firefox or Safari is a relatively straightforward process. (Not that doing so is usually necessary: the upgrade rate for non-IE browsers is fast, usually relieving you from the necessity of supporting more than several versions back.)
The ideal solution is to have different versions of IE running on virtual machines within your operating system.
An increasingly favoured solution is to use an online virtual environment, such as browsershots.org, browserstack or CrossBrowserTesting. Generally speaking these tools are easier to use than installing a suite of browsers on your own machine; they can also be used from any computer with an internet connection, and provide a wide coverage of different browsers and platforms. However, being internet services, rather than local programs, they are also somewhat slower in providing feedback. As a result, anticipating and fixing as many problems as you can in advance becomes important, rather than working on problems line-by-line and waiting for the online emulator to respond in order to check each one.
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