Case
Case is the one typographic decision that changes the words themselves rather than their setting. It is also where the web tells its most common lie, which is that it has small caps.
Capitals are spaced to sit beside lowercase letters, so a word set in capitals is always too tight until you open it. Small caps are a separate set of drawings — only three of the nine faces on this site carry them.
The revised edition, printed in Basel in 1932
The default, and correct for almost everything.
The revised edition, printed in Basel in 1932
Needs the tracking opened. Capitals were spaced to sit beside lowercase, not beside each other.
The revised edition, printed in Basel in 1932
Drawn at this size by the designer, with the stroke weight corrected to match the capitals.
THE REVISED EDITION, PRINTED IN BASEL IN 1932
Capitals scaled to 72 percent. The strokes thin with the scaling, so the line goes pale.
Applied at full strength to the capitals and at half to the small caps, which are shorter and need less. Somewhere around 0.08em is usually right for capitals.
Why capitals need tracking and lowercase does not
Every letter in a well-made face is spaced for the company it expects to keep. A capital H is drawn and spaced on the assumption that an o, an a or an e will usually follow it, and those letters bring their own side bearings. Put capitals next to each other and you have removed all of that built-in air at once. The word is now tighter than any pair in it was designed to be.
There is a second problem underneath the first. Lowercase text has a silhouette: ascenders stick up, descenders hang down, and the eye reads a good deal of a familiar word from its outline before it resolves a single letter. Capitals are all the same height, so the silhouette is a rectangle and that entire channel of information is gone. Tracking cannot give it back. It can only stop the letters crowding, which is why a long line of capitals stays slow to read no matter how carefully it is spaced.
The counterfeit
Real small caps are a separate set of drawings. The designer draws a capital at roughly the height of the lowercase x, then corrects the stroke weight, because a shape reduced to seventy percent has strokes at seventy percent and would otherwise sit pale next to the full capitals around it. That correction is the entire craft of the thing, and it is what you cannot get any other way.
When a font has no small caps, the browser makes them anyway: it takes the capital and scales it down. Nothing corrects the weight, so the result is thin, and set next to a real capital it looks washed out. The instrument above shows both in the same typeface, so the only variable is the feature. Of the nine faces on this site, exactly 3 carry a real smcp table: Bodoni Moda, Roboto Slab, Source Serif 4. The rest would be faked, silently, by any browser asked for small caps.
Worse, the faking is invisible to the person specifying it. The CSS is valid, no error appears, and the page looks approximately right at a glance. You only catch it by putting the real and the synthesised versions side by side, which is exactly what nobody does. If you want small caps, check that your font actually has them before you write the rule.
When each case is right
Sentence case for almost everything, including headlines, because it is the case the reader has the most practice with. Capitals for very short runs where the shape of the word does not matter and the signal of formality does — a two-word label, an abbreviation. Small caps for runs that need to be quieter than capitals but still marked: an acronym inside body text, a dateline, a byline, the first few words of a chapter opening.
The one rule worth memorising is about length. Case is a function of how many words you are setting. One word in capitals is a label. Six words in capitals is a decision the reader will pay for, and a whole sentence in capitals is a paragraph nobody finishes.
Track capitals up, track small caps half as much, and check that your small caps are real before you ask for them.