Although it may seem to be obvious for experienced web page authors, it was a discovery for me: XHTML does not understand special symbols “&” and “<” if written directly. So it’s impossible for example to perform “and” operation (&&) in javascript on the page.
There are two solutions for that:
1) Use CDATA for such sections of data, f.e.
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<script type="text/javascript">
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//< ![CDATA[
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function enableDisableRegisterButton(element1, element2) {
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registerButtonWrapper.disabled = !(element1.checked && element2.checked);
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...
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}
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]]>
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</script>