Now that the vertical attribute uses an enum type, DirectionSetting, according to WebIDL [1] Section 3.5 Enumerations:

"In the ECMAScript binding [of an Enumeration], assignment of an invalid string value to an attribute is ignored, while passing such a value as an operation argument results in an exception being thrown."

Consequently, the spec describing align needs to change

"If none of the values match, then the user agent must instead throw a SyntaxError exception."

to

"If none of the values match, then the user agent must ignore the attempt to set the attribute."

Note that I've asked the editor of WebIDL if perhaps the "strict" mode in ES should cause an exception rather than ignore the attempt, but have received no response yet.

[1] http://dev.w3.org/2006/webapi/WebIDL/