The opacity CSS property sets the opacity of an element. Opacity is the degree to which content behind an element is hidden, and is the opposite of transparency.
img {
opacity: 0.5;
}A <number> in the range 0.0 to 1.0, inclusive, or a <percentage> in the range 0% to 100%, inclusive, representing the opacity of the channel (that is, the value of its alpha channel). Any value outside the interval, though valid, is clamped to the nearest limit in the range.
Source: https://developer.mozilla.org/en-US/docs/Web/CSS/opacity