One page, five themes, zero bespoke CSS
Every control below reads its look from the route's registered theme: palette tokens for colour, component options for density, treatment and radius. Same markup, same classes, different theme boundary.
The page under the theme
Long-form content, headings and links, all reading theme tokens.
This page is the living proof for two documents: headless components own structure, and theming owns the look. Nothing between the hero and this sentence carries a site-authored style; every value resolves through a theme variable.
Why two layers
A component's structure — its tags, roles, label wiring — changes with the design system's semantics, not with its palette. Splitting the two means a re-skin is a theme edit, and an accessibility fix never re-opens a colour decision. The Button rebuild is the first component to ship on that split.
What the fixtures below prove
- Options, not palette. The "same palette" fixture changes only the option set; the colours are the page's own.
- Nesting by inheritance. Theme boundaries declare the option variables, component rules consume them, so A → B → A ends on A without a single descendant rule.
- Demand loading. The late fragment's stylesheet is not on the page until the runtime sees its marker.
A form with a real round trip
Server-validated. With the runtime: an island swap and focus lands on the summary. Without script: the same POST redirects back and this page re-renders the answer.
Field layouts under constraint
The dense route lays labels out inline; below the sheet's width, in a narrow container as much as a narrow viewport, the rows stack instead of overflowing.
A label this long wraps rather than widening its track.
The violation: 65536 is outside the range.
The rule: 1–65535.
A field whose control is an input group.
A whole field inside a group.
Every variant, every size
Variant is what a button means; treatment is how the theme draws it. One disabled, one with an icon, one external link.
Same palette, two option sets
The right panel is a nested ui.Themed whose theme keeps this page's tokens and changes only the options. Colour stays; density, treatment and radius flip.
This page's theme
Density, treatment and radius as registered for the route.
The other option set
Same palette, the flipped option set: the option compiler's work isolated from the tokens.
Nesting A → B → A
Three primary buttons. Each boundary declares the full option set; read the computed values and the innermost A matches the outer A.
Editorial
The page's theme: the outer A.
High contrast
The other registered theme, nested inside the page's scope.
Editorial again
The innermost scope redeclares the complete option set, so it wins by proximity: A → B → A ends on A.
Light, dark, system
The pill forces a scheme independent of the OS setting; Auto follows it. Scoped dark palettes flip with the document, not the wrapper.
Bare headless, styled ui
The left button is headless.Button with a nil Classes: structure, roles, no classes, no stylesheet. The right one is ui.Button on the same page. The banner beneath is headless.SystemBanner with its Strings resolved from the request through ui.StringsFor — English words here, the reader's locale the moment the site installs a translator.
headless.Button, nil Classes
ui.Button
headless.SystemBanner, ui.StringsFor(ctx)
Information: Strings from the request
The tone word and the dismiss name are headless.Strings fields; ui.StringsFor resolves them per request.
A cold LoadAuto insertion
Button is LoadAlways and proves nothing about demand loading. The fragment below arrives with a ui.Callout, whose sheet is LoadAuto and absent here: the runtime must fetch it when the marker appears.
Nothing loaded yet. The fragment's component stylesheet is not on this page.