When including a URL which contains an ampersand (“&”) a very common error occurs if it is not done properly.
<!-- This is invalid! --> <a href="foo.cgi?chapter=1§ion=2©=3&lang=en">...</a>
The above example returns 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 in some cases it may not be so. 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.
Note that when 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.