<html><head><meta http-equiv="Content-Type" content="text/html; charset=iso-885…
Many Document Types based on XML need a mandatory xmlns attribute on the root element. For example, the root element for XHTML might look like:
<html xmlns="http://www.w3.org/1999/xhtml">
…"bla.png" width="dreizehn>zentimeter" height="sonst<irgendeintext" alt="bla" />
This message may appear in several cases:
Below is the source input I used for this validation: