Nested a tags should be an error?

At the moment a document of this type:

```
<!DOCTYPE html>
<html>
<head>
<title>baba</title>
</head>
<body>
<a href='http://example.org'>mini<a href='http://example.org'>monkey</a></a>
</body>
</html>
```

does not produce errors using tidy 5.0.0. However, most online validators give an error with the double a tag above. I cannot decipher the HTML specification as to which is correct.
