Highlight — .ns-mark
The one rule that matters here: a highlighter marks a phrase, never a paragraph. Four tints exist so a page can mark two different things: shipped, deprecated, removed.
Real <mark> elements. Turn the stylesheet off and the emphasis survives.
Strike — .ns-strike
The correction pattern: Trailhead is the only way in there are three ways in.
Wrapped in <s>/<del>. text-decoration cannot animate, so the rule is drawn — which is also the only way to get the pen-stroke overshoot.
Circle — .ns-circle
Bulkify every trigger — the circle is for one phrase per screen.
An inline SVG path, not a border-radius pseudo-element: a real path can be wonky, and a perfect CSS ellipse always looks like a validation error.
Frame — .ns-frame
Governor limits RunLocalTests optional
Two pseudo-elements drawing two sides each, so the frame traces round the word instead of fading in finished. --brackets is the console read.
Scan line — .ns-scan
A CRT pass over display type. The band is masked so it brightens the glyphs rather than sitting on them — and it runs twice, then stops. Display sizes only: visible raster over body copy is a legibility problem.
Scramble — .ns-scramble
Characters settle left to right. It re-fires when a breakpoint re-wraps the line, so a responsive reflow reads as deliberate instead of as a jump. Without JS it is just text.