Can you style an active form input's label with just CSS

No. there is unfortunately no predecessor selector in css

input:focus -+ label { ... }

would be lovely.

having the label after the input would be dooable:

input:focus + label { ... }

you could use some positioning to display before...

UPDATED

Make sure you check the draft as this may change: https://drafts.csswg.org/selectors-4/#relational

The :has() relational pseudo-class will allow the selection of parents for example, the following selector matches only <a> elements that contain an <img> child:

a:has(> img)

This can be combined with other selectors such as :focus, :active or :not to offer a lot of potential.

Unfortunately browser support isn’t great at the time of writing: https://caniuse.com/#feat=css-has

Adding this for people finding this page in the future. CSS4 will have a parent selector allowing you to choose what element to apply the style to:

label! > input {
  font-weight: bold;
}

article > h1 + section > p! > b > a {
  font-style: italic;
}

For completeness, if your input field is within the label you can use focus-within:

HTML:

<label>
   <input name="example" type="text">
</label>

CSS:

label:focus-within {
   background: #DEF;
}