FontEnigma

LeMo aka PatternMan aka Frank E Blokland said:

if you type the alphabet in one direction, the obfusction in the opposite direction will become clear when you copy the result into a text editor. Evib~dvoo~hklggvw}

If you need to preserve the entered text so that it is available for copy-pasting, here's an idea that came to mind – with duplicating the text element – one for typing the text and one for displaying the font.

Here is the link to a test layout. Green cursor is the upper (operable) element – here you see only cursor. Blue text is the lower element with displayed font – the text is copied here with JavaScript.
https://jsfiddle.net/4p8zgfhx/

Explanation.

Place two text elements above each other (absolutely positioned inside the container), with the same font size and line height (to synchronise the caret position). Lower text element is the rendered font with obfuscation, that user will see. Upper text element is the transparent font without obfuscation (and with empty glyphs), that user operate. Top element should have transparent text but visible caret. Both fonts should have the same horizontal metrics (for displayed letter) to synchronise the caret. Then use JS to copy text from user-operable upper element to lower element.

John Savard said:

However, my understanding is that these days the most commonly used browsers don't support copy protection for web fonts any longer; and back when they did, each browser supported a different scheme. Absent a means of copy protection, one just looks at the page source, then the CSS file source if necessary, and downloads the font, which is within the abilities of a casual attacker.

There is a technique for loading a font as an encoded flow. It's good enough protection from a lazy attacker who just know how to save the file from a page cache (because now the font there is encoded), however it is still not a bulletproof approach. The problem is that the font customer (website owner) needs to do an extra steps for this (PHP, JS), and this is a huge stumbling stone. I would say, a stone unliftable for most. Not to mention that many people won't agree to the font not being stored in the cache but being dynamically loaded every time.

So all these discussed techniques can only give an initial effect on the font market / type tester website, until the font is purchased.
Usually, by that time the font has already been downloaded many times, most of which occur in the first few days.

Another idea that requires some technical resources (startup for some enthusiasts) is to have a database of malicious addresses (where fonts are usually leaked) and automatically scan it regularly for certain font names. It sounds crazy, but who knows what the future holds. In a better world where governments were concerned about this, some anti-crime agency would be handling all of this. In a better world.