Two Languages Are Created
Hiding From Old Browsers
The way the two languages work was just similar enough that it was possible to use browser sensing to work out whether the browser was Netscape or IE. The appropriate code for that browser could then be run. As the balance shifted toward IE gaining an equal share of the browser market with Netscape this incompatibility needed a resolution.
Before the DOM standard was complete both Netscape and Microsoft released their own versions. Netscape 4 came with its own document.layer DOM and Internet Explorer 4 came with its own document.all DOM. Both of these document object models were made obsolete when people ceased using either of those browsers as all browsers since then have implemented the standard DOM.