.busca {
    width: 13px;
    box-sizing: border-box;
border:0;
    outline: none;

    font-size: 16px;
    background-color: #343a40;
    background-image: url('https://www.w3schools.com/howto/searchicon.png');
    background-position: 10px 10px;
    background-repeat: no-repeat;
    padding: 12px 10px 12px 40px;
    -webkit-transition: width 0.4s ease-in-out;
    transition: width 0.4s ease-in-out;
    float:right;
}

.busca:focus {
    width: 300px;
    border-top: 0;
    border-right:0;
    border-left:0;
    background-color: #f8f9fa;
    border-bottom: 1px solid #ddd;
}