It's a CSS issue.
In your CSS, you have this selector .block-webform input.email
. Just add height: auto;
to it.
Actually you can type normally on the field, you can not just see what you typed due to CSS height limitation caused by limiting the height using this CSS property height: 1.8em;
.
I'm not able to tell you the lines' numbers because you compressed the CSS files :), but I believe you can easily find them.