Differences between Pure Internet Explorers and IE Compatibility Modes

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Or, yet more evidence that you should use feature detection instead of browser/user agent sniffing.

IE9

Not supported in IE9 but works in IE10-as-IE9

• Unprefixed CSS transform. Should require -ms-transform but doesn’t require it.

IE8

Supported in IE8 but not IE9-as-IE8

• ­ works in IE8 but does not work in IE9-as-IE8 Source

Supported in IE8 but not IE10-as-IE8

• The Matrix CSS filter. http://msdn.microsoft.com/en-us/library/ms533014(v=vs.85).aspx

Not supported in IE8 but works in IE9-as-IE8

• 32KB limitation on Data URIs: Test Case courtesy of @keeganstreet.
• SVG (yet IE9-as-IE8 displays <object data="file.svg">). Source
• Reliable reuse of array.sort arguments http://www.zachleat.com/web/array-sort/
• Navigation timing courtesy of @bluesmoon

IE7

Supported in IE7 but not IE8+-as-IE7

Not supported in IE7 but works in IE8+-as-IE7

• Web Storage courtesy of @mattdsteele

False Positives

• Modernizr reports: IE8 in compatibility view mode has onhashchange, but doesn't support it.
• Box sizing (not supported in IE7 but haz.io reports true in IE9-as-IE7)

Additional uncatalogued data

• https://docs.google.com/spreadsheet/ccc?key=0AntZUVoCEOyYdGQyMG1OWjg1THNFaWdkMHliX2FNc0E#gid=0 Thanks @dmolsen
• TODO: Search through modernizr for document.documentMode references.
• TODO: Compare haz.io results to caniuse results.

I remembered you were compiling this list and I ran into a couple things on my latest project that I didn't notice in IE10 as IE8 but rendered wrong in actual IE8. Maybe these are too specific for what you are looking for, but here they are anyways.

Twitter Bootstrap Breadcrumbs inside a row-fluid. This is supposedly an inline-block issue. I found the following fix:
.breadcrumb li {
margin-right: 1px;
}

Also select box dropdowns in actual IE8 are fixed width, but in IE10 as IE8 they are dynamic (extend to the length of your content).