style.css

* {
    margin: 0;
    padding: 0;
}
body {
    width: 100%;
    height: 100vh;
    overflow: hidden;
    position: relative;
}
#ground {
    bottom: 0;
    width: 100%;
    height: 100px;
    position: absolute;
    background-color: green;
}
#character {
    width: 50px;
    height: 50px;
    bottom: 100px;
    position: absolute;
    background-color: red;
    right: calc(100% - 100px);
}
.obstacle {
    right: 0;
    width: 30px;
    height: 100px;
    bottom: 100px;
    position: absolute;
    background-color: black;
}
.score {
    margin-top: 10px;
    margin-left: 10px;
    font-family: sans-serif;

    -webkit-user-select: none; /* Safari */        
    -moz-user-select: none; /* Firefox */
    -ms-user-select: none; /* IE10+/Edge */
    user-select: none; /* Standard */
}