When designing a website a Web Developer is responsible for testing on all browsers, not just internet explorer:-
As there are so many new mobile devices the number of new browsers increases every day.
Make sure your Website works on the browser used most by your customers.
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When developing a website I suggest designing for Firefox then fixing the site so it works on Internet Explorer and then any others.
This is the most efficient way to correct, as developing for Internet Explorer then correcting for the others takes twice as long.
There is no point in ignoring 50% of web users because your website doesn’t work correctly for their browser:-
In September 2002 approx. 83% used Internet Explorer, Firefox didn’t exist and the rest had 17%
In September 2008 approx 50% use IE and 44% use Firefox and the rest have 6%……more details