I have a hypothesis for what's causing the occasional black vote counts. The "answer-votes" and "answer-votes answered-accepted" divs occur within <a> tags, and I think Firefox is getting confused about whether to apply the color for things in <a> tags (which is almost black) or the color for the "answer-votes" div.
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<div class="answer-summary"><a href="/questions/8338/line-bundles-trivial-after-extension-of-the-base-field/8384#8384" title="total number of votes for this answer"><div class="answer-votes">11</div></a><div class="answer-link"><a href="/questions/8338/line-bundles-trivial-after-extension-of-the-base-field/8384#8384" class="answer-hyperlink">Line bundles trivial after extension of the base-field</a></div>
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<div class="answer-summary"><a href="/questions/1610/pairs-of-shortest-paths/1650#1650"><div class="answer-votes answered-accepted" title="total number of votes for this answer, which was accepted as the correct answer by the question owner">11</div></a><div class="answer-link"><a href="/questions/1610/pairs-of-shortest-paths/1650#1650" class="answer-hyperlink">Pairs of shortest paths </a></div>
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<div class="answer-summary"><a href="/questions/3270/which-rings-are-subrings-of-matrix-rings/3384#3384"><div class="answer-votes answered-accepted" title="total number of votes for 2 answers; one of which was accepted as the correct answer by the question owner">11</div></a><div class="answer-link"><a href="/questions/3270/which-rings-are-subrings-of-matrix-rings/3384#3384" class="answer-hyperlink">Which rings are subrings of matrix rings?</a> (2)</div>
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<div class="answer-summary"><a href="/questions/3278/whats-a-reasonable-category-that-is-not-locally-small/3282#3282"><div class="answer-votes answered-accepted" title="total number of votes for this answer, which was accepted as the correct answer by the question owner">11</div></a><div class="answer-link"><a href="/questions/3278/whats-a-reasonable-category-that-is-not-locally-small/3282#3282" class="answer-hyperlink">What’s a reasonable category that is not locally small?</a></div>
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Reloading the page makes the anomaly go away! It must be a bug in Firefox.
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edit (by Anton): I inserted the image here
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In the list of answers on the user page, you just get the number of votes the answer has received:

The box is green if the answer was accepted and light grey if it wasn't.
I'm not sure what you're referring to when you say "some of the vote counts are in bold while most are not" or when you say "some of the vote counts are black on green". Where should I look to see examples of these things?
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