@mixin video__in
    transition: opacity 0.1s 0.1s linear

@mixin video__out
    transition: opacity 0.1s linear

.video__sticky, .video__top-half
    z-index: 3

.video__top-half
    width: 100%
    max-width: $maxwidth
    background: black
    margin: 0 auto
    text-align: center

.video__sources
    color: darken($gray, 10%)
    background: lighten($lightgray, 10%)
    padding: 10px $padding
    text-align: center

.video__lines
    margin-top: 15px

.video__line
    margin-top: 10px
    .zh-cn
        margin: 0 0 0px

.video__line__translation
    position: relative
    button
        border: 1px solid $lightgray
        background: none
        color: $default

.video__line__translation__show
    position: absolute
    top: 0
    left: 0
    opacity: 0
    @include video__out

.video__line__translation__text
    position: relative
    z-index: 1
    @include video__in

.video__line__translation--hidden
    .video__line__translation__show
        opacity: 1
        z-index: 2
        @include video__in
    .video__line__translation__text
        opacity: 0
        cursor: default
        @include video__out

@mixin video__in
    transition: opacity 0.1s 0.1s linear

@mixin video__out
    transition: opacity 0.1s linear

.video__sticky, .video__top-half
    z-index: 3

.video__top-half
    width: 100%
    max-width: $maxwidth
    background: black
    margin: 0 auto
    text-align: center

.video__sources
    color: darken($gray, 10%)
    background: lighten($lightgray, 10%)
    padding: 10px $padding
    text-align: center

.video__lines
    margin-top: 15px

.video__line
    margin-top: 10px
    .zh-cn
        margin: 0 0 0px

.video__line__translation
    position: relative
    .btn
        border: 1px solid $lightgray
        background: none
        color: $default

.video__line__translation__show
    position: absolute
    top: 0
    left: 0
    opacity: 0
    @include video__out

.video__line__translation__text
    position: relative
    z-index: 1
    @include video__in

.video__line__translation--hidden
    .video__line__translation__show
        opacity: 1
        z-index: 2
        @include video__in
    .video__line__translation__text
        opacity: 0
        cursor: default
        @include video__out

Replace button selector by .btn
Replace button selector by .btn
