There are standards for how scripts and webpages should look on the internet. The main problem being that a vast majority of computer users uses a PC running Windows and Internet Explorer, and you're always more inclined to look at pages with your own computer with your browser. If it works there.. "It works!!!".
The main problem is that Microsoft for a very long time has walked their own path, which suits/works with their OS. When Java was released by Sun, Microsoft soon went ahead and released a variation of Java and got sued by Sun for doing that. The reason it works like that has to do with Microsoft adressing things in the OS, rather than let the Browser handle things. (Which was one of the reasons why Netscape sued Microsoft. Netscape, which aren't saints themself, comes with it's own Java-engine as an example, which supposedly should take care of things. Internet Explorer doesn't need that, since it uses features of the OS (something Netscape can't do) to carry out things.
That way, we have an Internet that might work or not, depending on what Operating System or which webbrowser you're using.
The thing is, there are often two ways of creating a thing on the net. A correct way, that are recognized by all Operating Systems and all webbrowsers (since all those understand all the standards). Then there's a variation of that, that works only in one browser or OS.
W3C is the organisation for an international web-standard, has released DOM Level 3, their newest guide/standard meant to unify all platforms and browsers. Especially since it's backed by most of the bigger companies with an interest in the development of the Internet, such as Microsoft, Sun, Macromedia etc.
Even if I doesn't think most people has any direct use of reading up on the standards, most users can be sure of using software and guides that follows the DOM Level 3 standards.
Now.. if they could work out a translator for cross-plattforming that would be a nice welcome. (Such as fonts being the same size. Fonts on a Macintosh, as an example, is only 3/4 of a font on a PC, as in.. If I specify the use of a Font size 12p, on a PC it will have about the same size as a 15p font on a Mac. That would be fixed by either having the size of the fonts as a percentage of the page-heigh, or you being able to define the font size for various platforms, as in.. if the OS is a Mac, use 14p, if it's a PC use 12p, or something. Not that you should have to write that out, but that the browsers recognize it automatically. Even if one way would be to have a script at start that detects the dpi (72 or 96) and adjusts the font according to what's used. "if ? font size x .75" sort of..