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DOCTYPEs that will lead to inconsistent behavior

(in browsers that support switching)

Unrecognized DOCTYPE

Browser Mode
NN6.01 Quirks
NN6.1 Quirks
NN6.2.x Quirks
Moz0.9.9 Strict
IE5/Mac Strict
IE6/Win Strict

Unrecognized DOCTYPE + URI

Browser Mode
NN6.01 Quirks
NN6.1 Quirks
NN6.2.x Quirks
Moz0.9.9 Strict
IE5/Mac Strict
IE6/Win Strict

HTML 4.0 Transitional + URI

Browser Mode
NN6.01 Quirks
NN6.1 Quirks
NN6.2.x Quirks
Moz0.9.9 Quirks
IE5/Mac Strict
IE6/Win Strict

HTML 4.0 Frameset + URI

Browser Mode
NN6.01 Quirks
NN6.1 Quirks
NN6.2.x Quirks
Moz0.9.9 Quirks
IE5/Mac Strict
IE6/Win Strict

HTML 4.01 Strict

Browser Mode
NN6.01 Strict
NN6.1 Strict
NN6.2.x Strict
Moz0.9.9 Strict
IE5/Mac Quirks
IE6/Win Strict

. Valid XHTML w/ XML

Browser Mode
NN6.01 Strict
NN6.1 Strict
NN6.2.x Strict
Moz0.9.9 Strict
IE5/Mac Strict
IE6/Win Quirks

Valid XHTML w/ XML + URI

Browser Mode
NN6.01 Strict
NN6.1 Strict
NN6.2.x Strict
Moz0.9.9 Strict
IE5/Mac Strict
IE6/Win Quirks

The following two DOCTYPEs may have inconsistent behavior because IE6/Win status is unknown in IE6/Win.

  1. Any XML
  2. Any XML + URI

Based on Eric A. Meyer's summary table: Mode switching in browsers

Note: DOCTYPEs that cause the newer browsers to switch into a "standards" mode try to render as closely as possible to W3C recommendations in order to deliver a more consistent cross-browser experience. "Quirks" mode attempts to emulate the broken behavior of the legacy browsers. It tries to render an HTML document in a manner like older browsers, e.g. Netscape 4, Internet Explorer 4, etc. Opera always attempts to render pages in standards–compliant mode.

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