Losslessly compress PNG files.

Before: 392 KB (402.156 bytes)
After:  247 KB (253.041 bytes)
Update index.html.

Add a doctype, charset, html lang and remove unneeded type for `<script>`.
Compress shield.jpg with kraken.io.

Before: 39163 bytes
After:  35517 bytes
Update index.html.

Switch to classes, add img alt text and dimensions.