Three main useful techniques to check input forms using only CSS

Technique1 : Blank check for input form

Using the 「placeholder-shown」 attribute

Result

Details

The attribute used in the CSS part of this input form is placeholder-shown.
This indicates when a placeholder is displayed.
In other words, by using 「not」 in the following specification, we can make the button clickable by increasing the transparency of the button when the placeholder is not displayed.

<input placeholder="Please enter a value.">
<button>save</button>

/*
Use opacity to reduce the initial transparency by half.
By using pointer-events, the initial button click is disabled.
*/
button {
    opacity: 0.5;
    pointer-events: none;
}

/*
Using not(:placeholder-shown), you can use
If you don't see the placeholder, you can use
Automatic setting of pointer-events.
Restore the opacity's transparency as well.
*/
input:not(:placeholder-shown) + button {
    opacity: 1;
    pointer-events: auto;
}

Technique2 : Check the format of the input form

Using the 「valid and invalid」 attribute

Result

Details

In the CSS part of this input form, I'm using invalid and valid as attributes.
This indicates if the form matches the type specified in the input tag's type attribute (valid) or not (invalid).
In other words, the following specification will change the background color depending on whether the input value matches the format.

<input type="email" placeholder="Enter email address">
<button>save</button>

/*
Using the invalid attribute, the
The background color if the type attribute format does apply.
*/
input:invalid {
  background-color: #ee6666;
}

/*
Using the valid attribute, the
The background color if the type attribute format does not apply.
*/
input:valid {
  background-color: #95f195;
}

Technique3 : Check the scope of the input form

Using the 「in-range and out-of-range」 attribute

Result

Details

The CSS parts of this input form use 「in-range」 and 「out-of-range」 attributes.
This indicates if the range of values specified in the input tag's min and max attributes is met (in-range) or not (out-of-range).
In other words, the following specification will mean that the background color will be changed depending on whether the input value is beyond the specified numerical range.

<input type="number" min="1" max="20">
<button>save</button>

/*
Using the in-range attribute, the
The background color if it falls within a range of values.
*/
input:in-range {
  background-color: #95f195;
}
/*
Using the out-of-range attribute, you can use the
Background color if the value does not fall within a range of values.
*/
input:out-of-range {
  background-color: #ee6666;
}