Why would you use a HTML/CSS width of 99.9% instead of 100%?

Question

I have seen many developers writing HTML or CSS inline style widths of 99.9% in places where I would use 100%. Is there any valid reason for using 99.9%? Does it have any effective difference from 100%?

EDIT to retweet MSalters' very good question: Considering that 99.9% is one pixel off above 500 pixels, why not 99.99%? I'd guess he's right, that if you're going with the dirty hack you should use 99.99%, does anyone disagree?

Additional References:

• @Michael Stum's comment about the CSS zoom property led me to this interesting link about triggering hasLayout with zoom: http://www.bennadel.com/blog/1354-The-Power-Of-ZOOM-Fixing-CSS-Issues-In-Internet-Explorer.htm.
• @Andrew Dunn offered this incredibly comprehensive link on hasLayout: http://www.satzansatz.de/cssd/onhavinglayout.html

Answer 1

The hasLayout property

It's a dirty hack used to set the IE specific hasLayout property of the element to true. The hasLayout property "determines how elements draw and bound their content, interact with and relate to other elements, and react on and transmit application/user events." Giving an element layout is an easy way to fix many layout related bugs which appear in Internet Explorer.

What's with the 99.9%?

Setting the width to 99.9% is one way to trigger it. The reason you would use 99.9% is because layout is given to an element if its width is set to anything other than auto. Setting it to a percentage prevents the need to use a fixed width.

After some testing in jsFiddle, I've come to the conclusion that it really isn't necessary to to use a width of 99.9%, using a width of 100% is just as effective. http://jsfiddle.net/3qfjW/2/ (IE-Only). It seems that setting width to 99.9% may have been a common misconception which stuck.. Spread the word people.

Other methods

You can also trigger hasLayout using zoom: 1; While this is the preferred method for many, as it doesn't mess with other style related features of an element, it is also invalid CSS code, which is not an option to use for some developers.

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Further Reading

For more methods to trigger hasLayout check out: http://www.satzansatz.de/cssd/onhavinglayout.html

For more information on the hasLayout property check out the MSDN article on hasLayout http://msdn.microsoft.com/en-us/library/bb250481(VS.85).aspx (This is actually a great read, lots of detailed information)

Answer 2

It's true width of anything but auto and zoom trigger hasLayout, and zoom is the more flexible property/value since it doesn't mess with the width but I don't think that's the reason why you saw the developers use 99.9%.

In certain floating cases it's necessary that the combined width of the floats do not add up to 100% ( if specified in percentages ) because IE6 incorrectly calculates that as beyond 100% and usually the latter float drops.

The solution is to either specify a -1px right margin just for IE or make the sum 99.99%.

I have documented the bug here. So whether it is related or not to what you were seeing, hope the bug link helps anyone experiencing it.