Syntax

• opacity: number (* strictly between 0 and 1) | inherit | initial | unset;

Remarks

If you do not want apply opacity, you can use this instead:

background: rgba(255, 255, 255, 0.6);

Resources:

• MDN: https://developer.mozilla.org/en/docs/Web/CSS/opacity;
• W3C Transparency: the ‘opacity’ property: https://www.w3.org/TR/css3-color/#transparency
• Browser support: http://caniuse.com/#feat=css-opacity