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Как сделать размытие при открытии popup окна на html?

@AndrewHaze:

И куда это вставлять?

spoiler

@import "compass/css3";

* { box-sizing: border-box; }
html, body {
  width: 100%; height: 100%;
  position: relative;
  margin: 0; padding: 0;
  font-family: 'Noto Sans', sans-serif;
  font-weight: 400;
  font-size: 16px; line-height: 22px;
  background-color: #F2E1AC;
}

.fancy-btn {
  -webkit-appearance: none;
  cursor: pointer; outline: none; border: none;
  position: relative; top: 50%;
  display: block; min-width: 10%;
  line-height: 55px; font-size: 14px;
  text-transform: uppercase;
  margin: 0 auto; padding: 0 10px;
  background-color: #F2594B;
  color: #FFF;
  border-radius: 3px;
  @include transform(translateY(-50%));
  &:hover {
    background-color: lighten(#F2594B, 5%);
  }
}

// completely redoing the modal from the ground up.
.modal-overlay {
    position: absolute;
    top: 0; left: 0; right: 0; bottom: 0;
    margin: auto;
    background-color: #fff;
    opacity: 0;
    visibility: hidden;
    z-index: 40;

    @include transition(opacity .25s ease 0s, visibility linear .35s);
    &.state-show {
        opacity: .7;
        visibility: visible;
        @include transition-delay(0s);
        @include transition-duration(.2s, 0s);
    }
}

.modal-frame {
    position: absolute;
    top: 0; left: 0; right: 0; bottom: 0;
    margin: auto;
    z-index: 50;

/*     display: table; */
    @include display-flex;
    @include align-items(center);
    -moz-box-align: center;
    -webkit-box-pack: center;
    -webkit-justify-content: center;
    justify-content: center;
    -moz-box-pack: center;
    -ms-flex-pack: center;
    
    width: 100%;
    text-align: center;
    visibility: hidden;

    // animation classes and behavior
    &.state-appear {
        visibility: visible;
        .modal-inset {
            @include animation(modalComeIn .25s ease);
            visibility: visible; /* to keep @ final state */
        }
        .modal-body {
            opacity: 1;
            @include transform(translateY(0) scale(1,1))
        }
    }
    &.state-leave {
        visibility: visible;
        .modal-inset {
            @include animation(modalHeadOut .35s ease .1s);
            visibility: visible;
        }
        .modal-body {
            opacity: 0;
            @include transition-delay(0s);
            @include transition-duration(.35s);
            @include transition-timing-function(ease);
            @include transform(translateY(25px))
        }
    }
}
@-moz-document url-prefix() {
    .modal-frame {
        height: calc(100% - 55px);
    }
}

.modal {
    display: block;
    vertical-align: middle;
    text-align: center;
}
    .modal-inset {
        position: relative;
        padding: 60px;
        background-color: white;
        min-width: 320px;
        min-height: 126px;
        margin: auto;
        visibility: hidden;
        @include box-shadow(2px 2px 8px 1px rgba(0,0,0,.2));
        @include backface-visibility(hidden); // helps render animation
        @include transform(translate3d(0,0,0)); // helps render animation
        @include transform-style(preserve-3d); // helps render animation
        .close {
            display: block;
            cursor: pointer;
            position: absolute;
            top: 10px;
            right: 10px;
            padding: 10px;
            opacity: .4;
            &:hover {
              opacity: 1;
            }
        }
    }
        .modal-body {
            margin: auto;
            opacity: 0;
            @include transform(translateY(0) scale(.8, .8));
            @include transition-property(opacity, transform);
            @include transition-duration(.25s);
            @include transition-delay(.1s);
            h3 {
              font-weight: 700;
              padding-bottom: 22px;
              display: block;
              color: #F2594B;
              text-align: center;
            }
            p {
              padding-bottom: 20px;
            }
            .ps {
              font-size: 12px;
              opacity: .3;
            }
        }

@mixin keyframes($name) {
  @-webkit-keyframes #{$name} {
    @content;
  }
  @-moz-keyframes #{$name} {
    @content;
  }
  @keyframes #{$name} {
    @content;
  }
}

@include keyframes(modalComeIn) {
    0% {
        visibility: hidden;
        opacity: 0;
        @include transform(scale(.8, .8));
    }
    65.5% {
        @include transform(scale(1.03, 1.03));
    }
    100% {
        visibility: visible;
        opacity: 1;
        @include transform(scale(1, 1));
    }
}

@include keyframes(modalHeadOut) {
    0% {
        visibility: visible;
        opacity: 1;
        @include transform(translateY(0) scale(1, 1));
    }
    100% {
        visibility: hidden;
        opacity: 0;
        @include transform(translateY(35px) scale(.97, .97));
    }
}

Как сделать размытие при открытии popup окна на html?

о существование этого фильтра я знаю, а как его использовать чтобы при открытии окна задний фон был размыт, а при закрытии размытие убиралось