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There have been several revisions of Javascript/ECMAScript/JScript, and different versions have been deployed in different situations. This table details which versions relate to which browser.
| Javascript | Mozilla | JScript | IE | ECMA |
|---|---|---|---|---|
| JavaScript 1.0 | Netscape 2 | JScript 1.0 | Early IE 3 | |
| JavaScript 1.1 | Netscape 3 | JScript 2.0 | Late IE 3 | ECMAScript 1 plus switch |
| JavaScript 1.2 | Netscape 4 | - | - | |
| JavaScript 1.3 | Netscape 4.5 | JScript 3.0 | IE 4 | |
| JavaScript 1.4 | Server only | - | - | |
| JavaScript 1.5 | Mozilla/Netscape 6 | JScript 5 | IE 5 | ECMAScript 3 |
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IE does support JavaScript 1.5, but it doesn't support it in the "language" attribute of the script tag.
If you include the language attribute (which was originally meant to allow a page to gracefully degrade in older browsers, a concept that's increasingly obsolete), you can't have a value greater than "javascript1.2". Anything besides "javascript" or "javascript1.2" and IE will likely not execute the script.