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Editorial Team
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Asked: 2026-05-31T02:13:41+00:00

Firefox has a certain tolerance when rendering bad HTML. This means even if a

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Firefox has a certain tolerance when rendering bad HTML. This means even if a closing tag is left out, the HTML will be displayed as if everything was fine. This tolerance aspect is particularly relevant when one is using JavaScript to manipulate or add content in the current page.

Since I use Firefox as my main testing/developing browser, I’ve been troubled more than once by this behavior of which the consequences are loss of functionality in “more strict” browsers. For instance the same code in Microsoft Internet Explorer failed to produce any visible output due to the mentioned missing tag on the added content.

Now the question is, is there any way of telling Firefox to be more strict about the accepted HTML and fail instead of “guessing a fix” for it (specially when the HTML is being added via JavaScript)?

PS: I’ve tried playing with the DOCTYPE but the results were the same.

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  1. Editorial Team

    All parsers are forgiving to some degree. Most mainstream sites have errors (not that this makes it excusable, just saying). If you develop with a debugger attached, you will catch script errors much more quickly. I also catch script errors by using a minifier in my build process (which will fail on major syntax issues). I valid my HTML markup using Visual Studio 2010’s real-time warnings (which aren’t always perfect) and periodically use the w3 validator service.

    For browser choice, I usually develop across the board; in one sitting I literally may switch between IE7/8/9 modes, Chrome, and Firefox. Safari and Opera usually work if the aforementioned browsers are covered. This way I don’t get too far down an erroneous path.

    BTW, DOCTYPE is important (even if it doesn’t always seem that way).

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