# Ambiguous

# Ambiguous

## Ambiguous

# NFC Å - Å

# NFKC Äffin - Äffin

# NFD Å - Å

# NFKD Äffin - Äffin

# HTML character reference in header - abcd

# 123 starts with numerals

# Mixed case Roman numerals - Ⅰ Ⅱ Ⅲ Ⅳ ⅰ ⅱ ⅲ ⅳ

# Extra syntax # {#custom id}

# ASCII -  !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_\`abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„…†‡ˆ‰Š‹ŒŽ‘’“”•–—˜™š›œžŸ ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ

# Non-ASCII - ZWNJ=&#x200c; Punctuation=&#x2042;

# Lower ASCII - \_\!\_\"\_\#\_\$\_\%\_\&\_\'\_\(\_\)\_\*\_\+\_\,\_\-\_\.\_\/\_

# Tab_&#9;_

#  Extra  spaces

# -dashes-

-----

It looks like Github auto ids

- Proceed top to bottom and use `sprintf("%s-%d", id, n)` when `id` has been previously auto-assigned where `n` is the smallest integer >= 1 such that that expression has not been previously seen.
- Codepoints < '0' are dropped except for spaces and `-`
- Punctuation and non-graphical codepoints >= '0' are dropped.
- Adjacent `-`s are not collapsed.
- HTML character references are decoded and encoded letters are not replaced with `-`.
- `{#...}` extension syntax is neither recognized nor exempted.
- IDs may start with ASCII numerals.
- Non-ASCII letters are not lower cased.
- No Unicode normalization is done.  See below.


```js
// Run in dev console to see what the auto-id assigner does to Normalization examples from unicode.org.
let ids = Array.from(document.querySelectorAll('*[id]')).map(x => x.id);
let idToIdentityNormalForms = {};
let normalForms = [ ['identity', x => x], ['NFC', x => x.normalize('NFC')], ['NFD', x => x.normalize('NFD')], ['NFKC', x => x.normalize('NFKC')], ['NFKD', x => x.normalize('NFKD')] ];

for (let id of ids) {
  let matches = [];
  for (let nf of normalForms) {
    if (nf[1](id) === id) { matches.push(nf[0]) }
  }
  idToIdentityNormalForms[id] = matches;
}
console.log(JSON.stringify(idToIdentityNormalForms, null, 2));

/*
produces
...
  "user-content-nfc-Å---Å": [
    "identity"
  ],
  "user-content-nfkc-Äffin---Äffin": [
    "identity",
    "NFC"
  ],
  "user-content-nfd-Å---å": [
    "identity"
  ],
  "user-content-nfkd-Äffin---äffin": [
    "identity"
  ],
...
*/
```