Blazor CSS Strategy

Blazor CSS Strategy (isolation-first hybrid)

We do not use Tailwind. The design system is hand-written plain CSS built on global design tokens. This skill is the standard for where component CSS lives and how to scope it.

It exists because Blazor CSS isolation has two hard walls that bite the moment a component composes FluentUI:

• Wall A — isolation scope. An isolated *.razor.css only stamps its scope attribute ([b-xxxxx]) on native HTML elements the component itself authors. A child component's root (even a light-DOM <FluentStack> div) never receives the scope attribute, so isolated CSS cannot target it. ::deep only helps when there is a scoped native ancestor to anchor on, and is slow/error-prone.
• Wall B — shadow DOM. FluentUI interactive primitives (fluent-button, fluent-text, fields, …) are web components with open shadow roots — in v4 and v5 alike. Their internals are reachable only via ::part() + CSS custom properties. No scoping strategy pierces the shadow boundary.

The rules

1. Default = Blazor CSS isolation (Foo.razor.css). An isolated component MUST render a native HTML root (<section>/<button>/<div>/<span>), never a FluentUI/child component as its root. This is what keeps isolation working.
2. Brand tokens are global in web-spa/wwwroot/css/tokens.css as CSS custom properties. Consume them with var(--twe-*). Tokens are the single source of truth for color, type scale, radius, elevation, and status palette — never hard-code these in component CSS.
3. Exception A — styling inside a FluentUI primitive (shadow DOM): use ::part() + CSS custom properties only. Nothing else works.
4. Exception B — styling a FluentUI light-DOM child you can't wrap, or runtime-dynamic CSS: own a scope handle and write a co-located <style> scoped to it; pass the handle to the FluentUI component via Class=. No ::deep. No inline style= (inline styles are prohibited under strict-CSP / locked-down browsers — reserve Style=@Value for genuinely dynamic per-instance values only).
   • Multi-instance component → scope by .@(Id) (the Id from the state base component, see Tiers below).
   • Singleton (e.g. the layout/shell) → a fixed namespaced root class (.twe-shell).

Two base-class tiers (keep them separate)

• Tier-1 — leaf primitives (Card, Button, StatusBadge, fields): inherit a thin ComponentBase derivative that provides only the attribute splat. No Id. Renders a native root → uses isolation (*.razor.css). Many instances, static styling — isolation is cheapest here.
• Tier-2 — state/container components (layout, shell, page composites, sections): inherit BaseComponent : TimeWarpStateDevComponent, which already exposes a public string Id. That Id is the Exception-B scope handle. Few instances, sometimes dynamic — scope-handle fits.

Canonical in-repo example (Tier-2 / Exception B)

web-spa/components/TimeWarpPage.razor (the app shell) does this — it renders FluentLayout / FluentNav / FluentTextInput (light-DOM children, Wall A) and styles them via a co-located <style> scoped to a fixed root class .twe-shell (the shell is a singleton, so a fixed class rather than .@(Id)):

<FluentLayout Class=@($"{Id} twe-shell")> … <FluentNav Class="twe-nav"/> … </FluentLayout>

<style>
  @(@"
    .twe-shell .twe-nav { background: var(--twe-paper-2); border-right: 1px solid var(--twe-rule); }
    .twe-shell .twe-appbar__search fluent-text-input { width: 100%; }
  ")
</style>

.twe-shell .twe-nav reaches the FluentNav's light-DOM root (a plain descendant selector from the Id'd ancestor — no ::deep, no wrapper div). Use a verbatim string @(@"…") so CSS braces are literal; only use the interpolated @($@"… {{ }} …") form when you need {Id}.

Tier-1 example (leaf, isolation)

@* Card.razor *@
<section @attributes="Attributes" class="@CssClass">
  <div class="twe-card__body">@ChildContent</div>
</section>

/* Card.razor.css */
.twe-card {
  background: var(--twe-paper);
  border: 1px solid var(--twe-rule);
  border-radius: var(--twe-radius);
}

Decision quick-reference

Notes

• FluentUI v5 did not remove Wall A or Wall B (verified empirically). The strategy is the same across v4 and v5.
• Reference implementation: the crunchit web-spa (Crunchitfs/crunchit branch Cramer/2026-05-29/initial, source/web-spa) runs this on FluentUI v5 — see its tokens.css, MainLayout.razor (.crunchit-shell), and Card.razor(.css).