<button style="">Hover Me</button>

button {
    position: relative;
    height: 60px;
    width: 200px;
    border: none;
    outline: none;
    color: #fff;
    background-color: #111;
    cursor: pointer;
    border-radius: 5px;
    font-size: 18px;
    font-family: 'Roboto', sans-serif;
}

* {
    margin: 0;
    padding: 0;
}
user agent stylesheet
button {
    appearance: auto;
    writing-mode: horizontal-tb !important;
    font-style: ;
    font-variant-ligatures: ;
    font-variant-caps: ;
    font-variant-numeric: ;
    font-variant-east-asian: ;
    font-weight: ;
    font-stretch: ;
    font-size: ;
    font-family: ;
    text-rendering: auto;
    color: buttontext;
    letter-spacing: normal;
    word-spacing: normal;
    line-height: normal;
    text-transform: none;
    text-indent: 0px;
    text-shadow: none;
    display: inline-block;
    text-align: center;
    align-items: flex-start;
    cursor: default;
    box-sizing: border-box;
    background-color: buttonface;
    margin: 0em;
    padding: 1px 6px;
    border-width: 2px;
    border-style: outset;
    border-color: buttonborder;
    border-image: initial;
}
button::before {
    position: absolute;
    content: '';
    top: -2px;
    left: -2px;
    height: calc(100% + 4px);
    width: calc(100% + 4px);
    border-radius: 5px;
    z-index: -1;
    opacity: 0;
    -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=0)";
    filter: url(data:image/svg+xml;charset=utf-8,<svg xmlns="http://www.w3.org/2000/svg"><filter id="filter"><feGaussianBlur stdDeviation="5" /></filter></svg>#filter);
    -webkit-filter: blur(5px);
    filter: blur(5px);
    background: -webkit-linear-gradient(45deg, #ff0000, #ff7300, #fffb00, #48ff00, #00ffd5, #002bff, #7a00ff, #ff00c8, #ff0000);
    background: linear-gradient(45deg, #ff0000, #ff7300, #fffb00, #48ff00, #00ffd5, #002bff, #7a00ff, #ff00c8, #ff0000);
    background-size: 400%;
    -webkit-transition: opacity .3s ease-in-out;
    transition: opacity .3s ease-in-out;
    -webkit-animation: animate 20s linear infinite;
    animation: animate 20s linear infinite;
}