In case you aren’t using ajax, or are still on JSF 1.x, and you really need to invoke a business action in cancel() method (i.e. just reloading the page is insufficient), then your best bet is to add immediate="true" to the button. This way all input fields which don’t have immediate="true" will be skipped in processing.
<h:commandButton value="Cancel" action="#{bean.cancel}" immediate="true" />
On JSF 2.x, much better is to submit the form by <f:ajax>, which by default only processes @this instead of @form.
<h:commandButton value="Cancel" action="#{bean.cancel}">
<f:ajax />
</h:commandButton>
If you want to navigate to another page here, add ?faces-redirect=true to the outcome in the cancel() method.
Or, if you actually don’t need to invoke any business action at all, then just use <h:button> wherein you directly specify the (implicit) navigation case outcome.
<h:button value="Cancel" outcome="previouspage" />
This will basically reload the page by a GET request. The <h:button> doesn’t exist in JSF 1.x, but you can also just use plain HTML+JS for that.