Javascript Madness: Layout Engines

In other pages in this series, I refer to most browser by their browser name ("Internet Explorer", "Opera", "Safari", "Konqueror") but instead of talking about "Firefox" I talk about "Gecko". This page is mainly here to explain that choice.

Layout Engines

Notes:

• This list is very far from complete. All of these layout engines are used in other browsers not listed here, and there are a few other layout engines.
• Apple's WebKit is derived from KHTML, but there are enough differences that programmers should probably think of them as separate. This may be changing, since some people say KHTML is being merged into webKit but don't hold your breath.
• Google's Chrome browser uses WebKit, but uses a different Javascript engine, so it usually behaves just like Safari, but not always.
• Netscape 8.1 had a menu option from which you could choose to render either with the Trident layout engine or the Gecko layout engine. Netscape 9 was Gecko-only.
• Maxthon 3 can switch between the Trident and Webkit.

In writing articles about the behavior of different versions of different browsers, it's generally clearest to refer to the browser instead of the layout engine, as layout engines are much less well known. However, since there are several fairly common Mozilla browsers with different version numbering schemes, this gets a bit awkward. I could say that there are certain annoying layout bugs in Firefox 1.0, Mozilla 1.7, Netscape 7.2 and 8.1, Camino 0.8 and Epiphany 1.4 and 1.8. But I'd much rather say that there are certain annoying layout bugs in Gecko 1.7, which underlies all those browsers.

This goes beyond ease of expression. In many cases, the only way to write "portable" Javascript code is to sense the browser type by looking at the navigator.user_agent string, and running different code for different versions of different browsers. If you do this, what you want to look for with the Mozilla browsers is not the browser version number, but the Gecko version number. For example, the following user agent string is for "Firefox 1.5" but the Gecko version is 1.8:

   Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8) Gecko/20051111 Firefox/1.5

Do not be confused by browser strings like this:

   Mozilla/5.0 (Windows NT 6.1; WOW64; rv:5.0) Gecko/20100101 Firefox/5.0

All the Gecko-based browsers except Firefox seem to be steadily faging away, so this distinction seems to becoming moot, but now there are a lot of WebKit browsers, and the same rule applies -- for Safari, you want to parse out the AppleWebKit version number, not the Safari version number.

Gecko Versions

Notes:

• Wikipedia's Gecko article contains a more detailed version of this table.
• While the Mozilla browser still existed under that name, it's version numbers always matched the version numbers of the rendering engine. That association obviously no longer holds.
• Starting with Firefox 5.0, Gecko version numbers have been reset to always match Firefox version numbers.
• Development of the Netscape and Galeon browsers has been discontinued. The Epiphany browser has dropped Gecko and is using WebKit instead.
• Gecko 1.8 was the first not to piss me off. Browsers based on older versions should be avoided.
• Gecko 1.5 and 1.2 pretty much never saw the light of day. They were never in wide usage.
• Netscape 8.1 could be set to use either Gecko or Trident, the Windows layout engine. It did send different user agent strings depending on which layout engine was currently selected. Netscape 8.1 said it used Gecko 1.7, but it behaved in many ways more like Gecko 1.8. I think they must have applied many Gecko 1.8 patches to it.
• The mapping from Gecko versions to Epiphany and Galeon version numbers is approximate. The same browser version may be built with any of several different Gecko versions in different Linux distributions. The Galeon version numbers are the earliest releases compatible with the final Gecko release.

It should be noted that there was a time when some of the Mozilla people denied that Gecko was named "Gecko". They say that that is term invented by Netscape's marketing department for the combination of the Mozilla "NGLayout" project (which was previously called "Raptor") and the Mozilla "XPFE" project. Which doesn't explain why the user agent strings in all Mozilla browsers say "Gecko". So to heck with them. It's "Gecko" whether they like it or not. Much better name anyway. Sometimes marketing departments are not wrong.

WebKit Versions

Actually, the mapping between Safari version numbers and WebKit versions numbers isn't all that rigid either. It is quite possible to have several WebKit versions installed on your computer simultaneously, and to tell whatever version of Safari you are using to use any one of them. However, only serious browser geeks do this, so you needn't worry about it.

Note that Apple and Google hardly ever release the exact same version of WebKit. It looks like Google tends to use young versions of the newest branch, while Apple uses more mature versions of an older branch. This is probably an artifact of their radically different release cycles. To some degree, Google may be doing Apple's beta testing.

Chrome version numbers are weird. They are like two independent version numbering schemes concatinated together. The first two numbers look like the marketing department's version numbers, what the version number would look like if their release cycle was as slow as Apple's. The last number just seems to steadily tick upward as new revisions are checked into the release build, entirely independently of the first two digits. I suspect that if you want to have your software check Chrome version numbers, then you should ignore the first two digits, and use only the third and forth digit.