This list is based on scanning through the comments in Bootstrap's Less source code and the various warnings in Bootstrap's docs.

// In Mobile Safari, date inputs require a pixel line-height that matches the
// given height of the input.

• twbs/bootstrap#11266
  • @mdo: "the comment seems to imply unitless values are busted"
• twbs/bootstrap#13099

label {
  display: inline-block;
  max-width: 100%; // Force IE8 to wrap long content
}

• See twbs/bootstrap#13141
• No idea as to the cause; hasLayout maybe???
  • http://www.sitepoint.com/web-foundations/internet-explorer-haslayout-property/

Seems there is a bug in IE7-8 where input[type="text"] & <textarea> that are in a container which has filter: alpha(opacity=N); are not re-rendered when being typed in.

Curiously enough, if you move your mouse out of the parent with the filter, the text will magically appear / update.

• See twbs/bootstrap#3552
• http://jsbin.com/pisagahi/1

• twbs/bootstrap#9239
• h5bp/html5-boilerplate#984 (comment)

// IE9 fubars the placeholder attribute in text inputs and the arrows on
// select elements in input groups. To fix it, we float the input.

• Details: twbs/bootstrap#11561 (comment)

// IE8-9 hack for event handling
//
// Internet Explorer 8-9 does not support clicks on elements without a set
// `background-color`. We cannot use `filter` since that's not viewed as a
// background color by the browser. Thus, a hack is needed.
//
// For IE8, we set solid black as it doesn't support `rgba()`. For IE9, we
// set alpha transparency for the best results possible.
background-color: #000 \9; // IE8
background-color: rgba(0,0,0,0); // IE9

• twbs/bootstrap#11186
• https://github.com/twbs/bootstrap/commit/6a93a6b88a4b874fba5a1d1edd817cbd91ccfacc
• Explicit background-color: transparent; does not help
• filter: alpha(opacity=0); alone does not help
• opacity: 0; alone does not help
  • is not supported in IE8
  • is supported in IE9, but setting only an opacity doesn't avoid the bug
• IE9: opacity + non-transparent background-color works as a fix
• rgba(0,0,0,0)
  • is not supported in IE8
  • works as a fix in IE9
• IE8-9: filter: alpha(opacity=0); + non-transparent background-color works as a fix
• Need to test background: transparent

The following incompatibilities have been successfully documented in MDN:

• <fieldset> min-width legacy weirdness
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/fieldset$compare?to=589697&from=584707
• resize requires overflow other than visible
  • Is already documented in MDN. The Bootstrap source code comment was in error.
  • Also filed CSSLint/csslint#517
• iOS Safari [disabled] opacity
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Input$compare?to=588017&from=572223
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/textarea$compare?to=588343&from=587245
• Android Browser <select> menu indicator triangle bug
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/select$compare?to=590135&from=590133
• Viewport device-width bug in IE10 on Windows 8 and Windows Phone 8
  • https://developer.mozilla.org/en-US/docs/Web/CSS/@viewport$compare?to=592567&from=592565
• Firefox placeholder text opacity
  • https://developer.mozilla.org/en-US/docs/Web/CSS/::-moz-placeholder$compare?to=587215&from=522761
• Firefox for Android gradients on form controls
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/button$compare?to=587233&from=584697
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/select$compare?to=587235&from=536155
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/textarea$compare?to=587243&from=578897
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Input$compare?to=588303&from=588017
• Firefox for Android border on input[type="file"]
  • https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Input$compare?to=588351&from=588303

Hopefully these edits will survive in some form and not get wholesale reverted.