一、“融情于景”1.0

<form action="" class="search-bar"><input type="search" name="search" pattern=".*\S.*" required><button class="search-btn" type="submit"><span>Search</span></button>
</form>

* {border: 0;box-sizing: border-box;margin: 0;padding: 0;
}
:root {font-size: calc(16px + (24 - 16)*(100vw - 320px)/(1920 - 320));
}
body, button, input {font: 1em Hind, sans-serif;line-height: 1.5em;
}
body, input {color: #171717;
}
body, .search-bar {display: flex;
}
body {background: #f1f1f1;height: 100vh;
}
.search-bar input,
.search-btn, 
.search-btn:before, 
.search-btn:after {transition: all 0.25s ease-out;
}
.search-bar input,
.search-btn {width: 3em;height: 3em;
}
.search-bar input:invalid:not(:focus),
.search-btn {cursor: pointer;
}
.search-bar,
.search-bar input:focus,
.search-bar input:valid  {width: 100%;
}
.search-bar input:focus,
.search-bar input:not(:focus) + .search-btn:focus {outline: transparent;
}
.search-bar {margin: auto;padding: 1.5em;justify-content: center;max-width: 30em;
}
.search-bar input {background: transparent;border-radius: 1.5em;box-shadow: 0 0 0 0.4em #171717 inset;padding: 0.75em;transform: translate(0.5em,0.5em) scale(0.5);transform-origin: 100% 0;-webkit-appearance: none;-moz-appearance: none;appearance: none;
}
.search-bar input::-webkit-search-decoration {-webkit-appearance: none;
}
.search-bar input:focus,
.search-bar input:valid {background: #fff;border-radius: 0.375em 0 0 0.375em;box-shadow: 0 0 0 0.1em #d9d9d9 inset;transform: scale(1);
}
.search-btn {background: #171717;border-radius: 0 0.75em 0.75em 0 / 0 1.5em 1.5em 0;padding: 0.75em;position: relative;transform: translate(0.25em,0.25em) rotate(45deg) scale(0.25,0.125);transform-origin: 0 50%;
}
.search-btn:before, 
.search-btn:after {content: "";display: block;opacity: 0;position: absolute;
}
.search-btn:before {border-radius: 50%;box-shadow: 0 0 0 0.2em #f1f1f1 inset;top: 0.75em;left: 0.75em;width: 1.2em;height: 1.2em;
}
.search-btn:after {background: #f1f1f1;border-radius: 0 0.25em 0.25em 0;top: 51%;left: 51%;width: 0.75em;height: 0.25em;transform: translate(0.2em,0) rotate(45deg);transform-origin: 0 50%;
}
.search-btn span {display: inline-block;overflow: hidden;width: 1px;height: 1px;
}/* Active state */
.search-bar input:focus + .search-btn,
.search-bar input:valid + .search-btn {background: #2762f3;border-radius: 0 0.375em 0.375em 0;transform: scale(1);
}
.search-bar input:focus + .search-btn:before, 
.search-bar input:focus + .search-btn:after,
.search-bar input:valid + .search-btn:before, 
.search-bar input:valid + .search-btn:after {opacity: 1;
}
.search-bar input:focus + .search-btn:hover,
.search-bar input:valid + .search-btn:hover,
.search-bar input:valid:not(:focus) + .search-btn:focus {background: #0c48db;
}
.search-bar input:focus + .search-btn:active,
.search-bar input:valid + .search-btn:active {transform: translateY(1px);
}@media screen and (prefers-color-scheme: dark) {body, input {color: #f1f1f1;}body {background: #171717;}.search-bar input {box-shadow: 0 0 0 0.4em #f1f1f1 inset;}.search-bar input:focus,.search-bar input:valid {background: #3d3d3d;box-shadow: 0 0 0 0.1em #3d3d3d inset;}.search-btn {background: #f1f1f1;}
}

二、“融情于景”2.0

<div class="container"><input type="text" placeholder="Search..."><div class="search"></div>
</div>

* {margin: 0;padding: 0;box-sizing: border-box;
}html, body {width: 100%;height: 100%;
}body {background: #252525;
}.container {position: absolute;margin: auto;top: 0;left: 0;right: 0;bottom: 0;width: 300px;height: 100px;
}
.container .search {position: absolute;margin: auto;top: 0;right: 0;bottom: 0;left: 0;width: 80px;height: 80px;background: crimson;border-radius: 50%;transition: all 1s;z-index: 4;box-shadow: 0 0 25px 0 rgba(0, 0, 0, 0.4);
}
.container .search:hover {cursor: pointer;
}
.container .search::before {content: "";position: absolute;margin: auto;top: 22px;right: 0;bottom: 0;left: 22px;width: 12px;height: 2px;background: white;transform: rotate(45deg);transition: all 0.5s;
}
.container .search::after {content: "";position: absolute;margin: auto;top: -5px;right: 0;bottom: 0;left: -5px;width: 25px;height: 25px;border-radius: 50%;border: 2px solid white;transition: all 0.5s;
}
.container input {font-family: "Inconsolata", monospace;position: absolute;margin: auto;top: 0;right: 0;bottom: 0;left: 0;width: 50px;height: 50px;outline: none;border: none;background: crimson;color: white;text-shadow: 0 0 10px crimson;padding: 0 80px 0 20px;border-radius: 30px;box-shadow: 0 0 25px 0 crimson, 0 20px 25px 0 rgba(0, 0, 0, 0.2);transition: all 1s;opacity: 0;z-index: 5;font-weight: bolder;letter-spacing: 0.1em;
}
.container input:hover {cursor: pointer;
}
.container input:focus {width: 300px;opacity: 1;cursor: text;
}
.container input:focus ~ .search {right: -250px;background: #151515;z-index: 6;
}
.container input:focus ~ .search::before {top: 0;left: 0;width: 25px;
}
.container input:focus ~ .search::after {top: 0;left: 0;width: 25px;height: 2px;border: none;background: white;border-radius: 0%;transform: rotate(-45deg);
}
.container input::placeholder {color: white;opacity: 0.5;font-weight: bolder;
}