2.2.5 [HTML] Section 6.16, Frame target names
frame target names (%FrameTarget; in the DTD) must begin with an alphabetic character. User agents should ignore all other target names.
All Document Modes (All Versions)
The value of the target attribute for a FRAME element does not have to begin with an alphabetic character [a-zA-Z]. A value is not ignored if it begins with a non-alphabetic character.
If the name of the target frame begins with an alphanumeric [a-zA-Z0-9] character, the specified content is loaded into the target frame. If it begins with a non-alphanumeric character, a new browser window is opened.