Scanlines

A cathode ray tube drew its picture with a single electron beam sweeping left to right in horizontal lines, and on an interlaced set only half of those lines were painted per pass, so the gaps between them stayed visibly darker. That is the whole effect being imitated: a few dark rules, one or two pixels apart, laid over everything. On the web it is one repeating-linear-gradient on an absolutely positioned layer with pointer-events: none, which is why it shows up in so many retro kits. The rest of the costume is optional and usually present anyway: a vignette darkening the corners, a soft bloom around bright text, a slight barrel curve, a faint horizontal offset on the colour channels, and a slow flicker in the overall brightness.

Scanlines are an overlay, not a style. That is the line worth holding when the two get confused: the terminal aesthetic is a whole costume with a monospace grid, one phosphor colour, and a prompt, and scanlines are one layer that can be dropped on top of any of it, or on a photograph, or on a perfectly ordinary dashboard. The same layer is a member of two other families. VHS decoration adds tracking noise, a rolling distortion band, and torn chroma, imitating tape rather than the tube. Cassette futurism, the retrofitted future of Alien and Blade Runner, uses the same raster as its screen texture while everything else is chunky amber hardware.

The tuning is finer than it looks. Lines have to be spaced against the reader’s device pixels, not against CSS pixels, or a two pixel period becomes a moire on a high density screen and a solid grey wash when the page is zoomed. A dark raster is subtractive: it takes real contrast out of every glyph underneath it, so text has to be brightened to pay for the effect, and the overlay should be checked with the contrast checker rather than with the eye. The flicker is the risky part, since a full frame brightness change is exactly the pattern that photosensitivity guidance is about, so keep the amplitude small, keep it well under three flashes a second, and drop it entirely for a reader who has asked for reduced motion. And the whole layer is decoration: it belongs behind aria-hidden, out of the pointer’s way, and it should never be the only thing carrying meaning.