Fixed HTML validation errors

index.html is now valid HTML5. Changes include:

* Replaced two redundant, incorrect \<meta\> tags with a single valid one
* Added empty "alt" attribute to an \<img\> tag that was missing it
* Removed redundant closing \</img\> tag
* Closed unclosed \<progress\> and \<label\> tags
* Replaced duplicate "id" attribute on a \<span\> element with a unique one