JSFiddle - React, Tailwind, and code Playground

HTML

<input id="id_sq_pl" name="sq_pl" value="" type="number" />

CSS

5. -> 5
.5 -> 0.5
5.568 -> 5.6
as -> alert
empty -> will be ok
spacebar -> empty, will be ok

JavaScript

$(window).load(function(){
    $('#id_sq_pl').blur( function(){
        var val = $(this).val();
        if(!val.length) return false;
        val = +val.replace(',','.');
        if(!val) {$(this).val('');alert('Введите правильное число'); return false};
        val = Math.round( val * 10 ) / 10;
        $(this).val((''+val).replace('.',','))
        console.log(val);
    });
});