& is HTML for "Start of a character reference".
& is the character reference for "An ampersand".
¤t; is not a standard character reference and so is an error (browsers may try to perform error recovery but you should not depend on this).
If you used a character reference for a real character (e.g. ™) then it (™) would appear in the URL instead of the string you wanted.
(Note that depending on the version of HTML you use, you may have to end a character reference with a ;, which is why &trade= will be treated as ™. HTML 4 allows it to be ommited if the next character is a non-word character (such as =) but some browsers (Hello Internet Explorer) have issues with this).
HTML doesn't recognize the & but it will recognize & because it is equal to & in HTML
Another 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"
Explanation:
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.
So the goal here is to avoid problems when you are trying to validate your website. So you should be replacing your ampersands with & when writing a URL in your markup.
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.