The error unexpected 'x' after 'DESCENDANT_SELECTOR' (Nokogiri::CSS::SyntaxError) (where x may be basically any character) occurs when the Nokogiri parser receives an invalid selector like .field_with_errors # or td <strong>.

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In Cucumber, the culprit will be an invalid step definition that builds an invalid selector:

# inside some step definition:
field = find_field(label)
page.send(expectation, have_css(".field_with_errors ##{field[:id]}"))

The above raises the mentioned error if field[:id] is nil, i.e. the found field has no id.

Fix

Ensure your step definition cannot generate invalid selectors and instead prints better error messages. Adding field.should be_present might help,

Note

This error may be caused when your selenium browser window is too narrow, so that your driver can't see the field. May sound funny but true.