JSFiddle - React, Tailwind, and code Playground

HTML

<form name="telpruefen" method="post" action="">
       		<input type="text" name="telpruefen" id="telpruefen" placeholder="Telefonnummer eintragen" style="width: 100%; height: 3em; padding: 1.3em; text-align: center;font-size: 1.5em; border: 1px solid #efefef; background-color: #efefef;">
            <input type="submit" name="abschicken_telpruefen" id="abschicken_telpruefen" value="Telefonnummer prüfen" class="btn_telpruefen">
        </form>

CSS

#abschicken_telpruefen {
  margin: 1em 0px 0px;
  width: 100%;
  border: none;
  border-image-source: initial;
  border-image-slice: initial;
  border-image-width: initial;
  border-image-outset: initial;
  border-image-repeat: initial;
  padding: 0.5em;
  background-color: rgba(239, 239, 239, 0.498039);
}

*::-webkit-input-placeholder {
    color: rgba(0, 102, 0, 0.5);
}
*::-moz-placeholder {
    /* FF 19+ */
    color:  rgba(239, 239, 239, 0.6);
}
*:-ms-input-placeholder {
    /* IE 10+ */
    color:  rgba(239, 239, 239, 0.6);
}