Answer principles round 1: 1 🔴, 5 🟡 (0 misread); the document carry's line feeds checked, element spellings shared with the writer, carried JSON bounded by type, an edit to a carry refused, one test per case, a start tag read one attribute at a time

src/html/block-elements.ts

@@ -0,0 +1,30 @@
+import type { JsonValue } from '../json-value.ts'
+
+export type ContainerType = 'blockquote' | 'bulletList' | 'orderedList'
+
+// The element of each node whose blocks, or list items, stand a line each inside it.
+export const containerElements: Readonly<Record<ContainerType, string>> = { blockquote: 'blockquote', bulletList: 'ul', orderedList: 'ol' }
+const headingLevels: readonly unknown[] = [1, 2, 3, 4, 5, 6]
+export const headingElements: readonly string[] = headingLevels.map((level) => `h${level}`)
+
+// `undefined` where the level is none of 1 to 6, so the heading rides the carry.
+export function headingElement(level: JsonValue | undefined): string | undefined {
+  return headingLevels.includes(level) ? `h${level}` : undefined
+}
+
+// The level of `<h1>` to `<h6>`, 0 for any other element.
+export function headingLevel(name: string): number {
+  return headingElements.indexOf(name) + 1
+}
+
+// An ordered list's ` start="…"`, `''` where it holds no order; `undefined` where `start` cannot spell the order: one that is no safe integer, or -0.
+export function listStart(order: JsonValue | undefined): string | undefined {
+  if (order === undefined) return ''
+  return typeof order === 'number' && Number.isSafeInteger(order) && !Object.is(order, -0) ? ` start="${order}"` : undefined
+}
+
+// The order a `start` value spells; `undefined` where `listStart` writes no such value.
+export function readListStart(value: string): number | undefined {
+  const order = Number(value)
+  return listStart(order) === ` start="${value}"` ? order : undefined
+}