Html 那边界从何而来

使用以下代码，我try 显示一个带有html和css的搜索栏.

Html:

<div id="headerStuff_searchbar">
                <form action="idunnophpjet.php" method="get" id="headerStuff_searchbar_form">
                    <fieldset>
                        <input id="headerStuff_searchbar_field"type="text" placeholder="Suchbegriff eingeben" name="search">
                        <button id="headerStuff_searchbar_button"></button>
                    </fieldset>
                </form>
</div>

另请参阅以下内容:

#headerStuff_searchbar{
    position: absolute;
    top: 81px;
    left: 752px;
    width: 500px;
    height: 43px;
    z-index: 1002;
}
fieldset {
    display: block;
    margin-inline-start: 2px;
    margin-inline-end: 2px;
    padding-block-start: 0.35em;
    padding-inline-start: 0.75em;
    padding-inline-end: 0.75em;
    padding-block-end: 0.625em;
    min-inline-size: min-content;
}
#headerStuff_searchbar_field{
    width: 395px;
    margin-right: 3px;
    box-sizing: border-box;
    height: 41px;
    font-size: 12pt;
    font-family: sans-serif;
    color: #676767;
    background-color: white;
    padding: 10px;
    margin: 0;
    vertical-align: middle;
    border-style: solid;
    border-width: 1px;
}
#headerStuff_searchbar_button{
    background-position: 10px center;
    width: 41px;
    background-image: url("http://www.jacobson-gymnasium.de/search/icon.png");
    background-repeat: no-repeat;
    background-size: 16px 16px;
    box-sizing: border-box;
    font-size: 12px;
    font-family: sans-serif;
    line-height: 19px;
    height: 41px;
    color: white;
    background-color: rgb(255, 128,0);
    padding: 10px 10px 10px 29px;
    vertical-align: middle;
    cursor: pointer;
    border-style: solid;
    border-width: 1px 1px 1px 1px;

}

除了它周围的灰色边框外，所有的东西都像它应该显示的那样显示. Where is that border from? And how do i remove it? Picture of searchbar个

我看了看哪个元素适合这个形状，我想是<form>个元素.在它的css中，我没有定义任何东西. 我试图在元素周围添加边框，但它只是像往常一样添加了边框.