root = Card([header, amountText, currencyText, dueDateText, payNowBtn])
header = CardHeader("Invoice")
amountText = TextContent("Amount Due: $1,200", "large")
currencyText = TextContent("USD", "small")
dueDateText = TextContent("Due Date: Dec 31, 2025", "default")
payNowBtn = Button("Pay Now", null, "primary", "normal", "medium")