现实生活中，物体并不是突然启动或者停止， 当然也不可能一直保持匀速移动。就像我们 打开抽屉的过程那样，刚开始拉的那一下动作很快， 但是当抽屉被拉出来之后我们会不自觉的放慢动作。或是掉落在地板上的物体，一开始下降的速度很快， 接着就会在地板上来回反弹直到停止。
今天就来介绍一下Chrome 113+全新推出的linear 缓冲函数。
一、目前CSS缓冲函数的局限性
提到 CSS 运动缓冲函数，你可能会想到以下几种
- linear：线性
- steps：阶段
- ease-in：淡入，先慢后快
- ease-out：淡出，先慢后快
- ease-in-out：淡入淡出，先慢后快，再变慢
- cubic-bezier()：贝塞尔曲线
其中，cubic-bezier比较灵活，可以通过 4 个参数，来改变整个运动状态。
下面是一个大家可能都知道的网站，可以通过拖拽实时生成缓冲函数。
https://cubic-bezier.com/
效果如下：
可以说以上CSS 所有的内置缓冲函数都可以用cubic-bezier()来表示，比如：
cubic-bezier(0,0,1,1)
/*等同于*/
linear如下所示：
虽然cubic-bezier非常强大，可以做很多运动函数，比如在这个网站的前几行都可以用cubic-bezier来实现。
https://easings.net
但是，问题在于cubic-bezier表示 3 次贝塞尔曲线，仅仅只能控制两个拖拽点，所以无论怎么自定义，也无法定义成像这样的曲线。
而这些很多都是非常真实的物理缓冲函数。
为了解决这个问题，linear()应运而生，下面一起来了解一下。
二、快速了解 linear()
相对于之前linear，这里变成了一个函数，也就是可以传一些值进去。
linear(0 0%, 0.31 65.09%, 1 100%)在Chrome开发者工具中长这样。
其实就是一些关键点，每组坐标用百分比或者小数表示，使用逗号分隔，更重要的是，这些点是可以无限添加的，这就意味着你可以添加足够多的细节来实现任意的曲线，例如：
linear(0 0%, 0 2.27%, 0.02 4.53%, 0.04 6.8%, 0.06 9.07%, 0.1 11.33%, 0.14 13.6%, 0.25 18.15%, 0.39 22.7%, 0.56 27.25%, 0.77 31.8%, 1 36.35%, 0.89 40.9%, 0.85 43.18%, 0.81 45.45%, 0.79 47.72%, 0.77 50%, 0.75 52.27%, 0.75 54.55%, 0.75 56.82%, 0.77 59.1%, 0.79 61.38%, 0.81 63.65%, 0.85 65.93%, 0.89 68.2%, 1 72.7%, 0.97 74.98%, 0.95 77.25%, 0.94 79.53%, 0.94 81.8%, 0.94 84.08%, 0.95 86.35%, 0.97 88.63%, 1 90.9%, 0.99 93.18%, 0.98 95.45%, 0.99 97.73%, 1 100%)非常多的点，在Chrome开发者工具中效果如下：
这其实也是内置的一个效果，叫做 bounce，也就是弹性的效果，非常适合做小球落地弹起的效果，我们实际用一下看看。
.ball{
--ease: linear(0 0%, 0 2.27%, 0.02 4.53%, 0.04 6.8%, 0.06 9.07%, 0.1 11.33%, 0.14 13.6%, 0.25 18.15%, 0.39 22.7%, 0.56 27.25%, 0.77 31.8%, 1 36.35%, 0.89 40.9%, 0.85 43.18%, 0.81 45.45%, 0.79 47.72%, 0.77 50%, 0.75 52.27%, 0.75 54.55%, 0.75 56.82%, 0.77 59.1%, 0.79 61.38%, 0.81 63.65%, 0.85 65.93%, 0.89 68.2%, 1 72.7%, 0.97 74.98%, 0.95 77.25%, 0.94 79.53%, 0.94 81.8%, 0.94 84.08%, 0.95 86.35%, 0.97 88.63%, 1 90.9%, 0.99 93.18%, 0.98 95.45%, 0.99 97.73%, 1 100%);
margin: 0 auto;
width: 50px;
height: 50px;
border-radius: 100%;
background-color: #9747FF;
animation: drop 2s infinite var(--ease);
}
@keyframes drop {
50%, 100% {
transform: translateY(350px)
}
}实际效果如下：
是不是非常有弹性？
还可以配合水平分量，实现一个抛物线掉落地上的效果：
.ball{
animation: drop 2s infinite var(--ease), x 2s infinite ease-out;
animation-composition: add;
}
@keyframes drop {
50%, 100% {
transform: translateY(350px)
}
}
/*水平动画*/
@keyframes x {
60%, 100% {
transform: translateX(250px)
}
}注意，这里用到动画合成，确保水平和垂直方向上的动画不会干扰。
实际效果如下：
你可以可以访问以下在线链接（Chrome 113+）：
- CSS linear() (juejin.cn)[1]
- CSS linear() (codepen.io)[2]
三、linear() 的优势
linear()最大的优势就是可复用性，就像cubic-bezier()一样，可以作为一个全局通用的运动缓冲函数。
仔细想想看，在以前，弹性动画其实也可以用动画关键帧实现的，比如下面是一个弹性的位移关键帧。
@keyframes translate-easeOutBounce {
0% {
transform: translateX(0%);
}
12% {
transform: translateX(-10.89%);
}
24% {
transform: translateX(-43.56%);
}
36% {
transform: translateX(-98.01%);
}
54% {
transform: translateX(-75.02%);
}
74% {
transform: translateX(-98.37%);
}
82% {
transform: translateX(-93.75%);
}
92% {
transform: translateX(-99.34%);
}
96% {
transform: translateX(-98.46%);
}
100% {
transform: translateX(-100%);
}
}虽然也能实现类似的效果，但是局限性很大，比如，我想实现一个弹性的缩放效果，在之前可能又要定义一个类似的缩放关键帧，如下：
@keyframes scale-easeOutBounce {
0% {
transform: scale(1);
}
12% {
transform: scale(0.89);
}
24% {
transform: scale(0.56);
}
36% {
transform: scale(0.02);
}
54% {
transform: scale(0.25);
}
74% {
transform: scale(0.02);
}
82% {
transform: scale(0.06);
}
92% {
transform: scale(0.01);
}
96% {
transform: scale(0.02);
}
100% {
transform: scale(0);
}
}是不是非常繁琐？而且里面的值也不方便调整（比如位移我不想用百分比）。
但现在有了linear()就不必这么麻烦了，完全可以共用一个缓冲函数。
:root{
--ease: linear(0 0%, 0 2.27%, 0.02 4.53%, 0.04 6.8%, 0.06 9.07%, 0.1 11.33%, 0.14 13.6%, 0.25 18.15%, 0.39 22.7%, 0.56 27.25%, 0.77 31.8%, 1 36.35%, 0.89 40.9%, 0.85 43.18%, 0.81 45.45%, 0.79 47.72%, 0.77 50%, 0.75 52.27%, 0.75 54.55%, 0.75 56.82%, 0.77 59.1%, 0.79 61.38%, 0.81 63.65%, 0.85 65.93%, 0.89 68.2%, 1 72.7%, 0.97 74.98%, 0.95 77.25%, 0.94 79.53%, 0.94 81.8%, 0.94 84.08%, 0.95 86.35%, 0.97 88.63%, 1 90.9%, 0.99 93.18%, 0.98 95.45%, 0.99 97.73%, 1 100%);
}
.ball{
animation: drop 2s infinite var(--ease);
}
.square{
transform: scale(0);
animation: scale 2s infinite var(--ease);
}
@keyframes drop {
50%, 100% {
transform: translateY(350px)
}
}
@keyframes scale {
50%, 100% {
transform: scale(1)
}
}效果如下：
是不是完全同步的弹性效果？
你可以可以访问以下在线链接（Chrome 113+）：
- linear-bounce (juejin.cn)[3]
- linear-bounce (codepen.io)[4]
除此之外，linear()还能用在过渡transition上，这就意味着transition也能实现复杂的过渡的效果了，不完全依赖animation。
div{
transition: linear(....) 2s;
}这两者是完全通用的，就不多介绍了
四、一些生成工具推荐以及常用缓冲函数
这么生动、复杂的缓冲函数，手写是不现实的，需要借助工具来生成。
首先 Chrome 开发者工具中自带了一部分常用的，在timing-function中可以选择。
目前还比较少，后面应该会持续更新。
还有一个网站也能生成相关曲线，可以根据需要调整曲线的精细程度（取样点）。
https://linear-easing-generator.netlify.app/
效果如下：
虽然可以选择的比较少，但是你可以在左边的Input区域自定义函数，将任意的 JS运动曲线函数或者SVG路径转换成我们所需要的linear()。
实际中我们其实只需要一些常用的就行了，下面是 GSAP中的的 Easing效果，大家可以自行去感受每个动画的差异。
https://gsap.com/docs/v3/Eases
转换成对应linear()如下，可以当成是全局通用函数直接使用，够了够了。
:root {
--none: linear(0, 1);
--power1-in: linear( 0, 0.0039, 0.0156, 0.0352, 0.0625, 0.0977, 0.1407, 0.1914, 0.2499, 0.3164, 0.3906 62.5%, 0.5625, 0.7656, 1 );
--power1-out: linear( 0, 0.2342, 0.4374, 0.6093 37.49%, 0.6835, 0.7499, 0.8086, 0.8593, 0.9023, 0.9375, 0.9648, 0.9844, 0.9961, 1 );
--power1-in-out: linear( 0, 0.0027, 0.0106 7.29%, 0.0425, 0.0957, 0.1701 29.16%, 0.2477, 0.3401 41.23%, 0.5982 55.18%, 0.7044 61.56%, 0.7987, 0.875 75%, 0.9297, 0.9687, 0.9922, 1 );
--power2-in: linear( 0, 0.0014 11.11%, 0.0071 19.24%, 0.0188 26.6%, 0.037 33.33%, 0.0634 39.87%, 0.0978 46.07%, 0.1407 52.02%, 0.1925 57.74%, 0.2559 63.49%, 0.3295 69.07%, 0.4135 74.5%, 0.5083 79.81%, 0.6141 85%, 0.7312 90.09%, 1 );
--power2-out: linear( 0, 0.2688 9.91%, 0.3859 15%, 0.4917 20.19%, 0.5865 25.5%, 0.6705 30.93%, 0.7441 36.51%, 0.8075 42.26%, 0.8593 47.98%, 0.9022 53.93%, 0.9366 60.13%, 0.963 66.67%, 0.9812 73.4%, 0.9929 80.76%, 0.9986 88.89%, 1 );
--power2-in-out: linear( 0, 0.0036 9.62%, 0.0185 16.66%, 0.0489 23.03%, 0.0962 28.86%, 0.1705 34.93%, 0.269 40.66%, 0.3867 45.89%, 0.5833 52.95%, 0.683 57.05%, 0.7829 62.14%, 0.8621 67.46%, 0.8991 70.68%, 0.9299 74.03%, 0.9545 77.52%, 0.9735 81.21%, 0.9865 85%, 0.9949 89.15%, 1 );
--power3-in: linear( 0, 0.0039 25%, 0.0117 32.89%, 0.0248 39.68%, 0.0457 46.22%, 0.0743 52.21%, 0.1113 57.77%, 0.1575 63%, 0.218 68.33%, 0.2901 73.39%, 0.3745 78.23%, 0.4718 82.88%, 0.5827 87.37%, 0.7074 91.71%, 0.8462 95.91%, 1 );
--power3-out: linear( 0, 0.1538 4.09%, 0.2926 8.29%, 0.4173 12.63%, 0.5282 17.12%, 0.6255 21.77%, 0.7099 26.61%, 0.782 31.67%, 0.8425 37%, 0.8887 42.23%, 0.9257 47.79%, 0.9543 53.78%, 0.9752 60.32%, 0.9883 67.11%, 0.9961 75%, 1 );
--power3-in-out: linear( 0, 0.0029 13.8%, 0.0184 21.9%, 0.0339 25.51%, 0.0551 28.81%, 0.0827 31.88%, 0.1168 34.76%, 0.1962 39.57%, 0.3005 44.02%, 0.4084 47.53%, 0.6242 53.45%, 0.7493 57.93%, 0.8495 62.97%, 0.8888 65.67%, 0.9213 68.51%, 0.9629 73.9%, 0.9876 80.16%, 0.998 87.5%, 1 );
--power4-in: linear( 0, 0.0024 29.91%, 0.008 38.03%, 0.0179 44.72%, 0.035 51.16%, 0.0595 56.88%, 0.0922 62.08%, 0.1338 66.88%, 0.1914 71.85%, 0.262 76.5%, 0.3461 80.88%, 0.4447 85.04%, 0.5587 89.01%, 0.689 92.82%, 0.8359 96.48%, 1 );
--power4-out: linear( 0, 0.1641 3.52%, 0.311 7.18%, 0.4413 10.99%, 0.5553 14.96%, 0.6539 19.12%, 0.738 23.5%, 0.8086 28.15%, 0.8662 33.12%, 0.9078 37.92%, 0.9405 43.12%, 0.965 48.84%, 0.9821 55.28%, 0.992 61.97%, 0.9976 70.09%, 1 );
--power4-in-out: linear( 0, 0.0012 14.95%, 0.0089 22.36%, 0.0297 28.43%, 0.0668 33.43%, 0.0979 36.08%, 0.1363 38.55%, 0.2373 43.07%, 0.3675 47.01%, 0.5984 52.15%, 0.7121 55.23%, 0.8192 59.21%, 0.898 63.62%, 0.9297 66.23%, 0.9546 69.06%, 0.9733 72.17%, 0.9864 75.67%, 0.9982 83.73%, 1 );
--quad-in: linear( 0, 0.0039, 0.0156, 0.0352, 0.0625, 0.0977, 0.1407, 0.1914, 0.2499, 0.3164, 0.3906 62.5%, 0.5625, 0.7656, 1 );
--quad-out: linear( 0, 0.2342, 0.4374, 0.6093 37.49%, 0.6835, 0.7499, 0.8086, 0.8593, 0.9023, 0.9375, 0.9648, 0.9844, 0.9961, 1 );
--quad-in-out: linear( 0, 0.0027, 0.0106 7.29%, 0.0425, 0.0957, 0.1701 29.16%, 0.2477, 0.3401 41.23%, 0.5982 55.18%, 0.7044 61.56%, 0.7987, 0.875 75%, 0.9297, 0.9687, 0.9922, 1 );
--expo-in: linear( 0, 0.0085 31.26%, 0.0167 40.94%, 0.0289 48.86%, 0.0471 55.92%, 0.0717 61.99%, 0.1038 67.32%, 0.1443 72.07%, 0.1989 76.7%, 0.2659 80.89%, 0.3465 84.71%, 0.4419 88.22%, 0.554 91.48%, 0.6835 94.51%, 0.8316 97.34%, 1 );
--expo-out: linear( 0, 0.1684 2.66%, 0.3165 5.49%, 0.446 8.52%, 0.5581 11.78%, 0.6535 15.29%, 0.7341 19.11%, 0.8011 23.3%, 0.8557 27.93%, 0.8962 32.68%, 0.9283 38.01%, 0.9529 44.08%, 0.9711 51.14%, 0.9833 59.06%, 0.9915 68.74%, 1 );
--expo-in-out: linear( 0, 0.0053 17.18%, 0.0195 26.59%, 0.0326 30.31%, 0.0506 33.48%, 0.0744 36.25%, 0.1046 38.71%, 0.1798 42.62%, 0.2846 45.93%, 0.3991 48.37%, 0.6358 52.29%, 0.765 55.45%, 0.8622 59.3%, 0.8986 61.51%, 0.9279 63.97%, 0.9481 66.34%, 0.9641 69.01%, 0.9856 75.57%, 0.9957 84.37%, 1 );
--circ-in: linear( -0, 0.0048 9.8%, 0.0192 19.5%, 0.043 29.02%, 0.0761 38.26%, 0.1181 47.13%, 0.1685 55.56%, 0.227 63.44%, 0.2929 70.71%, 0.3656 77.3%, 0.4445 83.15%, 0.5285 88.19%, 0.6173 92.39%, 0.7099 95.7%, 0.805 98.08%, 0.9021 99.52%, 1 );
--circ-out: linear( 0, 0.0979 0.48%, 0.195 1.92%, 0.2901 4.3%, 0.3827 7.61%, 0.4715 11.81%, 0.5555 16.85%, 0.6344 22.7%, 0.7071 29.29%, 0.773 36.56%, 0.8315 44.44%, 0.8819 52.87%, 0.9239 61.74%, 0.957 70.98%, 0.9808 80.5%, 0.9952 90.2%, 1 );
--circ-in-out: linear( -0, 0.0033 5.75%, 0.0132 11.43%, 0.0296 16.95%, 0.0522 22.25%, 0.0808 27.25%, 0.1149 31.89%, 0.1542 36.11%, 0.1981 39.85%, 0.2779 44.79%, 0.3654 48.15%, 0.4422 49.66%, 0.5807 50.66%, 0.6769 53.24%, 0.7253 55.37%, 0.7714 58.01%, 0.8142 61.11%, 0.8536 64.65%, 0.9158 72.23%, 0.9619 80.87%, 0.9904 90.25%, 1 );
--sine-in: linear( 0, 0.0035, 0.0141 10.7%, 0.0318 16.09%, 0.0566 21.51%, 0.0885 26.98%, 0.1278 32.53%, 0.2288 43.93%, 0.3563 55.48%, 0.5171 67.92%, 0.7139 81.53%, 1 );
--sine-out: linear( 0, 0.2861 18.47%, 0.4829 32.08%, 0.6437 44.52%, 0.7712 56.07%, 0.8722 67.47%, 0.9115 73.02%, 0.9434 78.49%, 0.9682 83.91%, 0.9859 89.3%, 0.9965, 1 );
--sine-in-out: linear( 0, 0.007 5.35%, 0.0282 10.75%, 0.0638 16.26%, 0.1144 21.96%, 0.1833 28.16%, 0.2717 34.9%, 0.6868 62.19%, 0.775 68.54%, 0.8457 74.3%, 0.9141 81.07%, 0.9621 87.52%, 0.9905 93.8%, 1 );
--back-in: linear( 0, -0.0029 4.31%, -0.0119 9.02%, -0.0837 31.27%, -0.0954 36.64%, -0.0998 41.45%, -0.0951 47.03%, -0.079 52.25%, -0.051 57.19%, -0.0108 61.92%, 0.0515 67.19%, 0.1312 72.27%, 0.2286 77.18%, 0.3445 81.96%, 0.4792 86.62%, 0.633 91.17%, 0.8066 95.63%, 1 );
--back-out: linear( 0, 0.1934 4.37%, 0.367 8.83%, 0.5208 13.38%, 0.6555 18.04%, 0.7714 22.82%, 0.8688 27.73%, 0.9485 32.81%, 1.0108 38.08%, 1.051 42.81%, 1.079 47.75%, 1.0951 52.97%, 1.0998 58.55%, 1.0954 63.36%, 1.0837 68.73%, 1.0119 90.98%, 1.0029 95.69%, 1 );
--back-in-out: linear( 0, -0.0059 4.51%, -0.0418 15.63%, -0.0499 20.72%, -0.0476 23.51%, -0.0395 26.12%, -0.0255 28.59%, -0.0055 30.95%, 0.0281 33.76%, 0.0717 36.47%, 0.1901 41.63%, 0.344 46.32%, 0.6156 52.65%, 0.7495 56.35%, 0.8757 60.97%, 0.9663 65.85%, 1.0006 68.58%, 1.0256 71.43%, 1.0417 74.43%, 1.0493 77.65%, 1.0452 83.02%, 1.0077 94.8%, 1 );
--elastic-in: linear( 0, 0.0019 13.34%, -0.0056 27.76%, -0.0012 31.86%, 0.0147 39.29%, 0.0161 42.46%, 0.0039 46.74%, -0.0416 54.3%, -0.046 57.29%, -0.0357, -0.0122 61.67%, 0.1176 69.29%, 0.1302 70.79%, 0.1306 72.16%, 0.1088 74.09%, 0.059 75.99%, -0.0317 78.19%, -0.3151 83.8%, -0.3643 85.52%, -0.3726, -0.3705 87.06%, -0.3463, -0.2959 89.3%, -0.1144 91.51%, 0.7822 97.9%, 1 );
--elastic-out: linear( 0, 0.2178 2.1%, 1.1144 8.49%, 1.2959 10.7%, 1.3463 11.81%, 1.3705 12.94%, 1.3726, 1.3643 14.48%, 1.3151 16.2%, 1.0317 21.81%, 0.941 24.01%, 0.8912 25.91%, 0.8694 27.84%, 0.8698 29.21%, 0.8824 30.71%, 1.0122 38.33%, 1.0357, 1.046 42.71%, 1.0416 45.7%, 0.9961 53.26%, 0.9839 57.54%, 0.9853 60.71%, 1.0012 68.14%, 1.0056 72.24%, 0.9981 86.66%, 1 );
--elastic-in-out: linear( 0, -0.0028 13.88%, 0.0081 21.23%, 0.002 23.37%, -0.0208 27.14%, -0.023 28.64%, -0.0178, -0.0061 30.83%, 0.0588 34.64%, 0.0651 35.39%, 0.0653 36.07%, 0.0514, 0.0184 38.3%, -0.1687 42.21%, -0.1857 43.04%, -0.181 43.8%, -0.1297 44.93%, -0.0201 46.08%, 1.0518 54.2%, 1.1471, 1.1853 56.48%, 1.1821 57.25%, 1.1573 58.11%, 0.9709 62%, 0.9458, 0.9347 63.92%, 0.9349 64.61%, 0.9412 65.36%, 1.0061 69.17%, 1.0178, 1.023 71.36%, 1.0208 72.86%, 0.998 76.63%, 0.9919 78.77%, 1.0028 86.12%, 1 );
--bounce-in: linear( 0, 0.0117, 0.0156, 0.0117, 0, 0.0273, 0.0468, 0.0586, 0.0625, 0.0586, 0.0468, 0.0273, 0 27.27%, 0.1093, 0.1875 36.36%, 0.2148, 0.2343, 0.2461, 0.25, 0.2461, 0.2344, 0.2148 52.28%, 0.1875 54.55%, 0.1095, 0, 0.2341, 0.4375, 0.6092, 0.75, 0.8593, 0.9375 90.91%, 0.9648, 0.9843, 0.9961, 1 );
--bounce-out: linear( 0, 0.0039, 0.0157, 0.0352, 0.0625 9.09%, 0.1407, 0.25, 0.3908, 0.5625, 0.7654, 1, 0.8907, 0.8125 45.45%, 0.7852, 0.7657, 0.7539, 0.75, 0.7539, 0.7657, 0.7852, 0.8125 63.64%, 0.8905, 1 72.73%, 0.9727, 0.9532, 0.9414, 0.9375, 0.9414, 0.9531, 0.9726, 1, 0.9883, 0.9844, 0.9883, 1 );
--bounce-in-out: linear( 0, 0.0078, 0, 0.0235, 0.0313, 0.0235, 0.0001 13.63%, 0.0549 15.92%, 0.0938, 0.1172, 0.125, 0.1172, 0.0939 27.26%, 0.0554 29.51%, 0.0003 31.82%, 0.2192, 0.3751 40.91%, 0.4332, 0.4734 45.8%, 0.4947 48.12%, 0.5027 51.35%, 0.5153 53.19%, 0.5437, 0.5868 57.58%, 0.6579, 0.7504 62.87%, 0.9999 68.19%, 0.9453, 0.9061, 0.8828, 0.875, 0.8828, 0.9063, 0.9451 84.08%, 0.9999 86.37%, 0.9765, 0.9688, 0.9765, 1, 0.9922, 1 );
}五、兼容处理和总结
前面也提到了，兼容性并不好，需要 Chrome 113+，完整兼容性如下：
不过，我们也可以采取回退措施，对不支持的浏览器采用传统的cubic-bezier即可
:root {
--ease: linear(0, 0.0039, 0.0157, 0.0352, 0.0625 9.09%, 0.1407, 0.25, 0.3908, 0.5625, 0.7654, 1, 0.8907, 0.8125 45.45%, 0.7852, 0.7657, 0.7539, 0.75, 0.7539, 0.7657, 0.7852, 0.8125 63.64%, 0.8905, 1 72.73%, 0.9727, 0.9532, 0.9414, 0.9375, 0.9414, 0.9531, 0.9726, 1, 0.9883, 0.9844, 0.9883, 1);
}
@supports not(animation-timing-function: linear(0, 1)) {
:root {
--ease: cubic-bezier(0.76, 0, 0.24, 1);
}
}下面总结一下本文关键点：
- 现实生活中，物体并不是突然启动或者停止，也不可能一直保持匀速移动，这就是运动缓冲函数
- 现有的缓冲函数并不能模拟复杂的运动，所以才有了 linear()
- linear() 可以设置任意的关键点，这样可以模拟出任何真实的运动效果
- linear()最大的优势就是可复用性，就像cubic-bezier()一样，可以作为一个全局通用的运动缓冲函数。
- linear()比较复杂，通常需要借助工具来生成，或者直接用现成的
- 兼容性还不太好，但是可以借助@support采取回退措施
[1]CSS linear() (juejin.cn)： https://code.juejin.cn/pen/7309823856212967474。
[2]CSS linear() (codepen.io)： https://codepen.io/xboxyan/pen/abXXqyE。
[3]linear-bounce (juejin.cn)： https://code.juejin.cn/pen/7309828284056305679。
[4]linear-bounce (codepen.io)： https://codepen.io/xboxyan/pen/BaMMYMo。
