Before making your epub available to your readers,it is a good idea to validate your handiwork to ensure that it is a valid epub and will be readable on all readers/software
If you have manually edited your ePub you can use the IDPF's ePub Validatorhttp://validator.idpf.org
Sigil has its own validation built in. It will warn you of html errors as you go, but it is a good idea to run the validator as well. It is available under Tools on the menu bar. Just click on validate ePub with Flight Crew
If you do have errors you will need to fix them before you make your ePub available for downloading as it may not display properly in all ereader systems.