JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container_options">
                <fieldset class="main_options_plug1" >
                    <legend>Konfiguration des offenen Endes</legend>
                    <div id="main_dropdown_options_plug1"></div>
                </fieldset>
                <fieldset class="main_options_cable">
                    <legend>Verfügbare Optionen für Ihr Kabel</legend>
                    <div id="main_dropdown_options_cable">
                        <p>Bestimmen Sie die Kabellänge: </p>
                        <select id="options_length" onChange="checkForSpecialLength();"></select>
                        <div id="specialLength" style="display:none;">
                            <p> Geben Sie die Sonderlänge in cm ein: </p>
                            <!--<input id="specialLengthInput" type="text" name="specialLengthInput" value=""  />-->
                            <input type="range" id="specialLengthInput" min="0" max="1000" value="0" step="10" onclick="setSpecialLength(this.value);" onmousemove="setSpecialLength(this.value);"/>
                            <span id="specialLengthRange">0</span>
                        </div>

                        <p> Bestimmen Sie die Farbe des Kabels: </p>
                        <select id="options_colour">

                        </select>
                        <p> Bestimmen Sie die Verpackung: </p>
                        <select id="options_packaging">

                        </select>
                        <input type="submit" value="Seriennummer" onClick="getArticleNumber();"/>

                    </div>

                </fieldset>
                <fieldset class="main_options_plug2" >
                    <legend>Konfiguration des offenen Endes</legend>
                    <div id="main_dropdown_options_plug2"></div>
                </fieldset>
            </div>

CSS

div.container_options {
    margin: 15px;
    clear: both;
    width: 980px;
}


fieldset.main_options_plug1{
    width: 300px;
    border: 1px solid black;
    border-radius: 5px;
    margin-left: 0;
    margin-right: auto;
   /* float: left; */
    margin-top: 10px;
    
}

.main_options_plug1 legend {
    font-weight: bold;
}

.main_options_plug2{
    width: 300px;
    border: 1px solid black;
    border-radius: 5px;
    margin-left: auto;
    margin-right: 0;
/*    float: right;*/
    margin-top: 10px;
}

.main_options_plug2 legend {
    font-weight: bold;
}

.main_options_cable {
    width: 300px;
    border: 1px solid black;
    border-radius: 5px;
    margin: auto;
/*    float: right;
    float: left;*/
    margin-top: 10px;
}