You already know that inline styles are "bad practice." Inline styles aren't reusable like CSS in separate files is, and thus, inefficient bloat. Unless of course, when it isn't. There are some instances where inline styles make perfect sense. Perhaps you have an application where user's pick their favorite color, and then you set the background of the body to that. Using an inline style in that case is actually more efficient than external CSS, since it's specific to one user and one element.
Now let's say you want to animate to a value set in an inline style. Say you want to animate a progress bar. You start at zero, and need to go up to any arbitrary value. Perhaps a call to the server tells you how complete an upload is and you set the value from that.
In a post I did nearly a year ago, I lamented that you can't animate to an inline style. You can't declare a keyframe in inline styles and you don't know what final value to animate to in the external CSS. Alas I was wrong, as I didn't know about this bonafide little CSS trick.
<div style="width: 75%">Upload is 75% complete.</div>
Here's the trick: just omit the to or 100% declaration from the @keyframe: