Things to try:
1. Unzip the EPUB and open the relevant page in Chrome. (If Chrome doesn't show the underlining, then neither will many eBook readers, as many readers are based on WebKit, and Chrome is based on Blink, which forked from WebKit.)
2. Run CSSLint on the CSS file. (If the CSS file contains any actual parsing errors, many eBook readers—particularly those based on ADE/RMSDK—will ignore the stylesheet entirely.)
3. Run a validator on the EPUB. (And if the EPUB doesn't validate, many eBook readers will keel over.)
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