The web page I got that from (don't remember where) said that the result of assigning an empty
string to the style's display attribute was to "set it to the default value". I interpreted that to mean that CSS would then act as if there were no display attribute on the tr element's style. That in turn would mean it would choose a display attribute from a higher-level style definition, which might well be the one in the built-in HTML styles ("the default").