There is a common error occurs when including a URL which contains an ampersand (“&”):
<!– This is invalid! –> <a href=”foo.cgi?chapter=1§ion=2©=3&lang=en”>…</a>
This example generates an error for “unknown entity section” because the “&” is assumed to begin an entity reference. Browsers often recover safely from this kind of error, but real problems do occur in some cases. In this example, many browsers correctly convert ©=3 to ©=3, which may cause the link to fail. Since ⟨ is the HTML entity for the left-pointing angle bracket, some browsers also convert &lang=en to〈=en. And one old browser even finds the entity §, converting §ion=2 to §ion=2.
To avoid problems with both validators and browsers, always use & in place of & when writing URLs in HTML:
<a href=”foo.cgi?chapter=1&section=2&copy=3&lang=en”>…</a>
Note that replacing & with & is only done when writing the URL in HTML, where “&” is a special character (along with “<” and “>”). When writing the same URL in a plain text email message or in the location bar of your browser, you would use “&” and not “&”. With HTML, the browser translates “&” to “&” so the Web server would only see “&” and not “&” in the query string of the request.
