5秒経っても悩んでたら答えがわかるよ！！

例のアレ.css

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document url-prefix(http://www.arealme.com/japanese-vocabulary-size-test/ja) {
    .answer {
        background-color: #efe !important;
        transition: background-color 1s ease 5s !important;
    }
    li:hover .answer[value="1"] {
        background-color: #fbb !important;
    }
}