Correct grammatical error (#540)

The previous form of [this sentence](https://w3ctag.github.io/design-principles/#usecase-oriented-apis) was incomplete in some way.

I've made it shorter and maybe more grammatical.

Author: martinthomson

index.bs

@@ -2854,9 +2854,9 @@ the exact lifecycle and data structures of the underlying native APIs.
 When possible, consider flexibility for new hardware.
 
 This means newly proposed APIs should be designed
-with careful consideration on how they are intended to be used
+with regard to how they are intended to be used
 rather than how the underlying hardware, device, or native API
-available today.
+happens to operate today.
 
 <h3 id="hardware-is-scary">Be proactive about safety</h3>