from the tw bootstrap doc:

> Dismiss buttons
Mobile Safari and Mobile Opera browsers, in addition to the data-dismiss="alert" attribute, require an href="#" for the dismissal of alerts when using an <a> tag.

> <a href="#" class="close" data-dismiss="alert">&times;</a>
Alternatively, you may use a <button> element with the data attribute, which we have opted to do for our docs. When using <button>, you must include type="button" or your forms may not submit.

> <button type="button" class="close" data-dismiss="alert">&times;</button>

in case you would go for `button` instead of `a` as default, just make sure it's gonna work on mobile.