Get last applied CSS rule for width using javascript

Question

I need to get the last applied CSS rule for the width attribute on an HTML element using Javascript.

Example 1:

<input type="text" class="fullwidth"/>

input {
  width: 300px;
}

.fullwidth {
  width: 100%;
}

Example 2:

<input type="text" class="fullwidth halfwidth"/>

input {
  width: 300px;
}

.fullwidth {
  width: 100%;
}

.halfwidth {
  width: 50%;
}

I need to get 100% for the first case and 50% for the second one. Basically I need the following highlighted value:

I can't use the getComputedStyle method because it retrieves the computed width in pixels, but I need the same value in the CSS rule.

Is there a way to achieve it?

Thanks in advance