JSFiddle - React, Tailwind, and code Playground
HTML
<input pattern="\d+" required/>
<span>шт.</span>
CSS
span {
display: none;
}
input:valid + span {
display: inline;
}
<input pattern="\d+" required/>
<span>шт.</span>
span {
display: none;
}
input:valid + span {
display: inline;
}