When I am visiting the Ruby on Rails tag on Stack Overflow, I see only some questions as highlighted. Why so?
"Ruby on rails" is added in my favorite tag list. So it should highlight all the questions..
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When you're viewing the questions page under a favourite tag (as you are here), only questions that have another of your favourite tags as well are highlighted.
This holds even if you are viewing the questions under a combination of tags - there must be a further favourite tag for the question to be highlighted. So for me, when I view c# questions and then click asp.net over on the 'Related Tags' on the right, I get this list, and only questions with another of my favourite tags (above and beyond c# and asp.net) are highlighted.
The site does not highlight questions in a tag search unless they have another tag that you have also favorited. If it did highlight all of the questions tagged ruby-on-rails in your case, all of the questions would be highlighted. This would not emphasize any of the questions over any others, so the highlighting would be useless.