viewbox.css has invalid CSS property value

viewbox.css has invalid CSS property value
for `.viewbox-container .loader`

**`widows: 100%;`**

100% is not a valid value for widows property and not supported by Firefox at all: 
https://developer.mozilla.org/en-US/docs/Web/CSS/widows
https://www.w3schools.com/cssref/css3_pr_widows.asp

Consider to fix the CSS not using the `widows` property all together.