JavaScript – Why Do Browsers Leak Memory?

I think your problem is not that there are different javascript engines (although there are). The problem that you are describing (that you need different code to get the same functionality) is probably because of various browsers understanding of HTML and particularly their implementation of the DOM. The work around HTML5 hopefully address a number of the areas of concern (although not all). This is taken directly from the HTML5 draft spec

1.5 Design notes

This section is non-normative.

It must be admitted that many aspects of HTML appear at first glance to be nonsensical and inconsistent.

HTML, its supporting DOM APIs, as well as many of its supporting technologies, have been developed over a period of several decades by a wide array of people with different priorities who, in many cases, did not know of each other's existence.

Features have thus arisen from many sources, and have not always been designed in especially consistent ways. Furthermore, because of the unique characteristics of the Web, implementation bugs have often become de-facto, and now de-jure, standards, as content is often unintentionally written in ways that rely on them before they can be fixed.

I think that expresses the problem quite nicely.

Why is Microsoft® Internet Explorer® incompatible with other browsers and published standards?

1. To add features that other browsers don't have
2. When programmers write web pages that use IE-only features, the code will not work on other browsers. This encourages end-users to view those sites in IE (and thus more people use IE, and since it only runs on Windows, more people use Windows).
3. Microsoft tools (FrontPage is the most famous for this) tend to produce code that only works on IE, or works best on IE. While this promotes #2 above, it deserves its own bullet because the next developer to work on that web site or web application is now has to use the same Microsoft technologies that were originally used produce it, or have to rewrite the entire project in a different technology.

Corporate intranets are particularly vulnerable to this strategy because they provide an environment where all users are on the same browser (until a new version comes out). Also, if a site is not customer facing, there will be no budget for even the tiny cost of supporting multiple browsers. Over time, the legacy intranet sites at a company prevent that whole company from abandoning IE (or even a specific version of IE) for another browser.

Microsoft's approach is a double-edged sword. On the one hand, it has produced the XML/HTTP asynchronous request functionality which made AJAX possible (and other positive innovations) which other browsers quickly copied. But Microsoft also ends up producing versions of IE which are incompatible with each other, thus biting the very developers who played the most into Microsoft's plans.

My favorite example of intra-version IE incompatibility is Micrsoft's own IE6 Countdown campaign where they encourage people not to use their own browser. The theory being that Microsoft can encourage people to upgrade to a more recent version of IE, whereas others who shout "IE6 Must Die" are more likely to encourage users to use another brand of browser instead of IE.

In short, it's a fairly successful market-domination strategy by Microsoft which has sometimes hurt and sometimes helped the rest of the industry. For myself, I try to produce only validated, standards-compliant HTML that is tested on the latest version of any browser with 2% or more market share (plus the latest 3 versions of IE).

Microsoft and Internet Explorer are registered trademarks of Microsoft corporation in the US and/or other countries.