My website, as you can see, isn't very complicated and you can easily launch it on a random 486 machine. However, it still uses some features that some of the ancient browsers don't understand.
I'll try to order it by importance, but it's pretty hard.
First two versions of Internet Explorer don't send this one. Wikipedia says that 40% of web browsers in March 1996 do send it and it's a part of HTTP 1.1. It's crucial if you host multiple domains on one web server, otherwise it won't know which domain you want. In that case, I think the current nginx configuration just shows 404 error, but I'm not sure.
I could probably make it without framesets if I tried enough (and with some PHP), but it looks more vintage with them, so why not? First supported by Netscape 2.0 from March 1996, at least according to Wikipedia.
I don't want to be limited by codepages. I can sometimes write something in Polish with its diacritics, requiring me to use either Windows 1250 or ISO-8859-2, but, in the same file, I might use ellipsis, which is not a part of it. This is the reason why the minimal Opera version is 6.0 Beta 1. Opera 4 just ignores the charset header, while Opera 5 converts all the non-ASCII characters to a codepage and then renders it. This works if your OS' codepage matches the most popular codepage in your language, but if you have English Windows and try to render Polish, you get incorrect letters.
Not strictly required since my CSS isn't very complicated. However, without CSS, the website is usually black on grey, which isn't the most pleasant to look at. This is the reason why the minimal 32-bit Internet Explorer version is 4 Platform Preview 1 - IE3 doesn't make the background black.
For instance of <body>, I set the font to Trebuchet MS and, if it's missing, to sans-serif. Mozilla M8 gives up if the first font is unavailable, making text supposed to be monospace in a default overall font. This is fixed in M12, but these releases, until M15, can't finish loading the page for some reason, so M15 it is.
Truth be told, not even Internet Explorer 6 does this well, but its broken support is good enough if you don't care about anything with opacity smaller than 255. Mozilla M7 tries to interpret these as 24-bit, resulting in the images scretched horizontally, cut ¾ of the way and with incorrect colours.
Introduced, just like framesets, by Netscape 2.0, only used for the buttons on the About me page. Not that important, but this Sprunk button is pretty good!
I'm not really descriptive with this heading, am I? Netscape 4.79 from 2001(!) can't properly render these buttons on about page, it puts them in 7-9 rows even though nobody asked for this. I wonder how a code that's simply 10 times <a href="link"><img src="pic"></a> next to each other causes it to break.
Arachne, to this day, doesn't support UTF-8. It also ignores some of the styling: fonts aren't coloured even though I specified, it makes a blue border around the GIF buttons even though I wanted it to be black (effectively non-existent). It's admirable there's any web browser for DOS that makes you read my page to some degree, but it's also very lacking in some aspects.
There are 3 browsers on these: Internet Explorer 5.01, Netscape 4.08 and Opera 3.62. Netscape doesn't know how to properly render the main page, as I established earlier. Opera doesn't support UTF-8. IE doesn't show PNGs. The least broken is definitely IE, because I don't use many images on my website.
The last version of Visual C++ for these platforms is 4.1 (with Win32s on 3.1) and, as far as I'm concerned, no browser compatible with this compiler renders my page completely.