<iframe sandbox="" class="ic-iframe" title="Check Social Share Images" tabindex="-1" src="https://example.com"></iframe>
The validator reports:
The "sandbox" attribute for this "iframe" element (with class="lv-iframe") has an empty or missing value. The expected value is one of the following: an alphanumeric string, no value, or a string.
sandbox
Applies extra restrictions to the content in the frame. The value of the attribute can either be empty to apply all restrictions, or space-separated tokens to lift particular restrictions:
[various attribute values are then listed]
Wouldn't the Validator therefore be incorrect in reporting this as an error?