What's the "correct" semantic way to specify image height and width? In CSS... width:15px; or inline... <img width="15" ?

Semantically this is wrong.

Usually when people are talking about code being semantically correct, they're referring to the code that accurately describes something.

HTML elements have meaning. "Semantically correct" means that your elements mean what they are supposed to.

Semantically correct usage of elements means that you use them for what they are meant to be used for.

It means that you're calling something what it actually is.

The classic example is that if something is a table, it should contain rows and columns of data. To use that for layout is semantically incorrect - you're saying "this is a table" when it's not.

Fits the ideal behind HTML HTML stands for "HyperText Markup Language"; its purpose is to mark up, or label, your content. The more accurately you mark it up, the better. New elements are being introduced in HTML5 to more accurately label common web page parts, such as headers and footers.

You don’t need to do any tricky cursor stuff, because it’s all semantically wired up already.

always use real <label for="correct_input"> elements. Just that alone is a UX consideration all too many forms fail on