Design Shiny UI

Responsive, accessible Shiny UI → bslib + modern layouts + custom CSS.

Use When

• New Shiny UI from scratch
• Modernize app: fluidPage → bslib
• Brand theming (colors, fonts)
• Responsive across screens
• A11y improvements

In

• Required: App purpose + audience
• Required: Layout type (sidebar, navbar, fillable, dashboard)
• Optional: Brand colors + fonts
• Optional: Custom CSS/SCSS (default: bslib only)
• Optional: A11y reqs (WCAG lvl)

Do

Step 1: Pick Page Layout

bslib page constructors:

# Sidebar layout — most common for data apps
ui <- page_sidebar(
  title = "My App",
  sidebar = sidebar("Controls here"),
  "Main content here"
)

# Navbar layout — for multi-page apps
ui <- page_navbar(
  title = "My App",
  nav_panel("Tab 1", "Content 1"),
  nav_panel("Tab 2", "Content 2"),
  nav_spacer(),
  nav_item(actionButton("help", "Help"))
)

# Fillable layout — content fills available space
ui <- page_fillable(
  card(
    full_screen = TRUE,
    plotOutput("plot")
  )
)

# Dashboard layout — grid of value boxes and cards
ui <- page_sidebar(
  title = "Dashboard",
  sidebar = sidebar(open = "closed", "Filters"),
  layout_columns(
    fill = FALSE,
    value_box("Revenue", "$1.2M", theme = "primary"),
    value_box("Users", "4,521", theme = "success"),
    value_box("Uptime", "99.9%", theme = "info")
  ),
  layout_columns(
    card(plotOutput("chart1")),
    card(plotOutput("chart2"))
  )
)

→ Layout matches nav + content needs.

If err: layout off → check using page_sidebar() / page_navbar() (bslib) not fluidPage() / navbarPage() (base shiny). bslib versions → better defaults + theming.

Step 2: Config bslib Theme

my_theme <- bslib::bs_theme(
  version = 5,                      # Bootstrap 5
  bootswatch = "flatly",            # Optional preset theme
  bg = "#ffffff",                   # Background color
  fg = "#2c3e50",                   # Foreground (text) color
  primary = "#2c3e50",              # Primary brand color
  secondary = "#95a5a6",            # Secondary color
  success = "#18bc9c",
  info = "#3498db",
  warning = "#f39c12",
  danger = "#e74c3c",
  base_font = bslib::font_google("Source Sans Pro"),
  heading_font = bslib::font_google("Source Sans Pro", wght = 600),
  code_font = bslib::font_google("Fira Code"),
  "navbar-bg" = "#2c3e50"
)

ui <- page_sidebar(
  theme = my_theme,
  title = "Themed App",
  # ...
)

Interactive theme editor during dev:

bslib::bs_theme_preview(my_theme)

→ App renders w/ consistent brand colors + fonts + BS5 components.

If err: fonts don't load → check net (Google Fonts needs it) or sys fonts: font_collection("system-ui", "-apple-system", "Segoe UI"). Theme vars don't apply → check theme passed to page fn.

Step 3: Cards + Columns Layout

ui <- page_sidebar(
  theme = my_theme,
  title = "Analysis Dashboard",
  sidebar = sidebar(
    width = 300,
    title = "Filters",
    selectInput("dataset", "Dataset", choices = c("iris", "mtcars")),
    sliderInput("sample", "Sample %", 10, 100, 100, step = 10),
    hr(),
    actionButton("refresh", "Refresh", class = "btn-primary w-100")
  ),

  # KPI row — non-filling
  layout_columns(
    fill = FALSE,
    col_widths = c(4, 4, 4),
    value_box(
      title = "Observations",
      value = textOutput("n_obs"),
      showcase = bsicons::bs_icon("table"),
      theme = "primary"
    ),
    value_box(
      title = "Variables",
      value = textOutput("n_vars"),
      showcase = bsicons::bs_icon("columns-gap"),
      theme = "info"
    ),
    value_box(
      title = "Missing",
      value = textOutput("n_missing"),
      showcase = bsicons::bs_icon("exclamation-triangle"),
      theme = "warning"
    )
  ),

  # Main content row
  layout_columns(
    col_widths = c(8, 4),
    card(
      card_header("Distribution"),
      full_screen = TRUE,
      plotOutput("main_plot")
    ),
    card(
      card_header("Summary"),
      tableOutput("summary_table")
    )
  )
)

Key primitives:

• layout_columns() — responsive grid w/ col_widths
• card() — container + optional header/footer
• value_box() — KPI + icon + theme
• layout_sidebar() — nested sidebar in cards
• navset_card_tab() — tabbed cards

→ Responsive grid adapts to screen size.

If err: cols stack on wide screens → check col_widths sum = 12 (BS grid). Cards overlap → fill = FALSE on non-filling rows.

Step 4: Dynamic UI

server <- function(input, output, session) {
  output$dynamic_filters <- renderUI({
    data <- current_data()
    tagList(
      selectInput("col", "Column", choices = names(data)),
      if (is.numeric(data[[input$col]])) {
        sliderInput("range", "Range",
          min = min(data[[input$col]], na.rm = TRUE),
          max = max(data[[input$col]], na.rm = TRUE),
          value = range(data[[input$col]], na.rm = TRUE)
        )
      } else {
        selectInput("values", "Values",
          choices = unique(data[[input$col]]),
          multiple = TRUE
        )
      }
    )
  })

  # Conditional panels (no server round-trip)
  # In UI:
  # conditionalPanel(
  #   condition = "input.show_advanced == true",
  #   numericInput("alpha", "Alpha", 0.05)
  # )
}

→ UI updates based on user + data.

If err: dynamic UI flickers → conditionalPanel() (CSS) over renderUI() when poss. Dynamic inputs lose vals on re-render → session$sendInputMessage() restore state.

Step 5: Custom CSS/SCSS (Optional)

Beyond bslib theme vars:

# Inline CSS
ui <- page_sidebar(
  theme = my_theme,
  tags$head(tags$style(HTML("
    .sidebar { border-right: 2px solid var(--bs-primary); }
    .card-header { font-weight: 600; }
    .value-box .value { font-size: 2.5rem; }
  "))),
  # ...
)

# External CSS file (place in www/ directory)
ui <- page_sidebar(
  theme = my_theme,
  tags$head(tags$link(rel = "stylesheet", href = "custom.css")),
  # ...
)

SCSS + bslib:

my_theme <- bslib::bs_theme(version = 5) |>
  bslib::bs_add_rules(sass::sass_file("www/custom.scss"))

→ Custom styles applied w/o breaking bslib theming.

If err: custom CSS conflicts → use BS CSS vars (var(--bs-primary)) over hardcoded colors. Ensures theme changes propagate.

Step 6: A11y

# Add ARIA labels to inputs
selectInput("category", "Category",
  choices = c("A", "B", "C")
) |> tagAppendAttributes(`aria-describedby` = "category-help")

# Add alt text to plots
output$plot <- renderPlot({
  plot(data(), main = "Distribution of Values")
}, alt = "Histogram showing the distribution of selected values")

# Ensure sufficient color contrast in theme
my_theme <- bslib::bs_theme(
  version = 5,
  bg = "#ffffff",      # White background
  fg = "#212529"       # Dark text — 15.4:1 contrast ratio
)

# Use semantic HTML
tags$main(
  role = "main",
  tags$h1("Dashboard"),
  tags$section(
    `aria-label` = "Key Performance Indicators",
    layout_columns(
      # value boxes...
    )
  )
)

→ App meets WCAG 2.1 AA: contrast + keyboard + screen reader.

If err: test browser a11y audit (Lighthouse). Contrast → WebAIM checker. All interactive → keyboard-focusable.

Check

• Layout OK on desktop + mobile
• bslib theme consistent across components
• Value boxes → correct themes + icons
• Cards resize in grid
• Custom CSS → BS vars, not hardcoded
• Plots have alt text
• Contrast ≥ WCAG AA (4.5:1 text)
• Interactive → keyboard accessible

Traps

• Mix old + new Shiny UI: No fluidPage() w/ bslib. Use page_sidebar() / page_navbar() / page_fillable() only.
• Hardcoded colors: Use var(--bs-primary) over #2c3e50. Hardcoded → breaks on theme change.
• Missing fill = FALSE: Value box + summary rows shouldn't stretch. fill = FALSE.
• Google Fonts offline: Air-gapped → sys fonts or self-hosted font files over font_google().
• Ignore mobile: Test browser responsive. layout_columns auto-stacks, custom CSS may not.

→

• scaffold-shiny-app — initial setup + theme config
• build-shiny-module — modular UI components
• optimize-shiny-performance — perf-conscious render
• review-web-design — visual review: layout + typo + color
• review-ux-ui — usability + a11y review