Rich Text to HTML

Requirements

• Export richTextToHTML({text,ranges}) as named and default. ranges contain {start,end,style}, using half-open UTF-16 offsets and style bold, italic, or underline. Multiple marks may overlap; duplicates are idempotent.
• Render bold with strong, italic with em, and underline with u. Active marks nest in that fixed outer-to-inner order. At each boundary retain the common prefix, close obsolete tags from inner to outer, and open new tags from outer to inner. This defines the canonical output even for crossing ranges.
• Treat overlapping/adjacent ranges of the same mark as one continuous active region. Input order must not affect output. Ignore zero-length ranges and emit no empty tags. Unstyled text remains outside tags; empty text produces an empty string.
• Escape raw text characters &, <, >, double quote, and apostrophe as &amp;, &lt;, &gt;, &quot;, and &#39;. Treat input as raw text, including existing entity-looking sequences. Preserve all other characters, including whitespace and newlines; newlines are literal rather than br tags.
• Validate text, ranges array, supported style labels, integer offsets, and 0 <= start <= end <= text.length. Invalid input throws TypeError. Do not clamp or normalize invalid ranges. Do not mutate input; frozen ranges work. No HTML parser, DOM, arbitrary CSS, links, block elements, grapheme offsets, or editor UI is required.

Examples

Input: richTextToHTML({text:"abcd",ranges:[{start:0,end:3,style:"bold"},{start:1,end:4,style:"italic"}]})
Output: <strong>a<em>bc</em></strong><em>d</em>

Input: richTextToHTML({text:"<x>&",ranges:[{start:0,end:4,style:"underline"}]})
Output: <u>&lt;x&gt;&amp;</u>