In a way proprietary features are such features, which belong to the specifications, but implementations broke in purpose existing specifications. New MS IE and Netscape/ Mozilla browsers use DTD-switches, when with certain without DTD or with certain DTD new browsers work in some matters at the same way as older and more buggy browsers. Newer browsers support overall better CSS-specifications and they are intended to work in certain modes better according to the CSS and (X)HTML specifications than in another mode. Netscape/Mozilla calls the better mode as standard mode/ strict mode as opposite to quirks mode. Microsoft calls the better mode as standard-compliant mode, when the other mode is just a mode, where the standard-compliant mode have been turned off. The switching mechanism is in MS IE 6.0 for Windows, MS IE 5.0 for Mac and Netscape 6.x/ corresponding Mozilla browsers.

In MS IE browsers the swicth affects especially calculating width and height properties in MS IE browsers. The system works quite well except calculating the width property in tables. Concerning MS IE 6.0 for Windows I have about some CSS1-related matters a short list[S].

In MS IE 5.0 for Mac the DTD-switch affects also to the width attribute of the TABLE element. MS IE 5.0 for Mac handles in the standard-compliant mode the width attribute like the corresponding property, which is an error at the sight of HTML (I handle these matters also in the page 10[S]).

The switch cause also in MS IE into proprietary CSS. MS IE doesn't accept for example scroll bar properties in the standard-compliant mode. That's why pages, which are inside IFRAME and which have colored scroll bars, use DTD, which doesn't switch the standard-compliant mode on. Other pages work in new MS IE and Netscape/Mozilla according to strict/ standard-compliant modes.

I have found, read from web pages or got following information through e-mails about following effects (this list is not complete and I need help to get it better):

MS IE 6.0 for Windows MS IE 5.0 for Mac Netscape 6.x
width and height properties for generic block-level elements. Yes Yes
The width property for the TABLE element. According to Mozilla org. the basic table layout strategy handles widths differently in some way (I don't have found differencies). Yes Yes, but minimum effect
The width property for TD and TH elements together with table-layout:fixed Yes Yes
The need of display:inline-block for ordinary inline level elements together with width and height properties Yes
Background properties for table elements Yes
When tables have a border style of inset or outset, the border color is based on the background color of the table or of the nearest ancestor with non-transparent background. Yes
The empty-cells property defaults to hide in quirks mode but show. Yes
In non-standand mode table cells with a border have a minimum width of one pixel. Yes
The basic table layout strategy ignores padding (on what) in non-standard mode. Yes
In non-standard mode mode floated tables never move to the next "line" if they don't fit next to other floats, they just keep widening the page. Yes
font-size:xx-small - font-size:xx-large (look at Model8c.html[S]). Yes Yes, but minimal difference
The CSS parser accepts invalid names of id and class selectors Yes
The CSS parser accepts colors not beginning with #. Yes Yes
The CSS parser interprets unitless numbers as px (concering Netscape browsers except for font-size because that was what Netscape 4.x did, and except for line-height and any other properties where they have distinct meaning). Yes Yes
The fonts for button INPUT elements and SELECT elements are computed differently. Yes
Proprietary CSSYes

The list below is not exact because MS IE 6.0 and Netscape 6.x accept in non-standard mode different errors. Netscape has also much HTML element and attribute related matters, which I have not listed.

The switching point is different in MS IE and Netscape/Mozilla. In Netscape/ Mozilla browsers the strict mode starts from HTML 4.0 Strict or HTML 4.01 Transitional document types. In MS IE the standard-compliant mode starts from the HTML 4.0 Transitional document type, if the URL ("http://www.w3.org/TR/REC-html40/loose.dtd") has been given. If the URL has not been given standard-compliant mode starts from the HTML 4.0 Strict document type.

Microsoft: CSS Enhancements in Internet Explorer 6 Public Preview
Mozilla: Mozilla Quirks Mode Behavior.>

Note. The DTD-declaration must be on the top of the page without anything before it. I had in the CSS-site in some pages before it a comment, when MS IE understood XHTML 1.0 Transitional documents so that the standard-compliant mode was off.