(meter) min, max, low, high, optimum, value: R
(progress) max, value: R
DONE WITH FLOATS.

width, height: N
maxlength, minlength: N
rows: N⁺
rowspan: {x in N | x ≤ 65534}
cols: N⁺
colspan: {x in N⁺ | x ≤ 1000}
size: N⁺
tabindex: Z
DONE WITH INTEGERS.

step: R⁺ U {"any"}
  - This will be a one-off. I can either feed it a *float32 and count the null as
  many, or I can define a new `type StepValue *float32` with a couple of helper
  functions `StepAny() StepValue {return nil}`
  and `StepInt(value int) StepValue {return StepValue(&value)}`.
DONE WITH STEP.

EXPLICIT BOOLS:
contenteditable: explicit bool
draggable: explicit bool
spellcheck: explicit bool
DONE WITH EXPLICIT BOOLS.

accesskey: either a single printable character, or (poorly supported) a space-delimited list of characters
DONE WITH ACCESSKEY.

STRINGS:
action: URL (string)
alt: string
charset: IANA MIME encoding name (string)
cite: URI (string)
content: string
contextmenu: menu ID (string)
data: URL (string)
dirname: the the element's name ++ ".dir", or maybe any string; needs more research
download: destination filename (string)
form: ID of form (string)
formaction: URL (string)
href: URL (string)
hreflang: a string from the grammar specified in https://www.ietf.org/rfc/bcp/bcp47.txt
id: string
itemprop: string
label: string
lang: language tag specified in https://www.ietf.org/rfc/bcp/bcp47.txt (string)
list: ID of datalist (string)
media: CSS media query (string)
name: string
pattern: regex (string)
placeholder: string
poster: URL (string)
src: URL (string)
srcdoc: inline HTML (string)
srclang: language tag specified in https://www.ietf.org/rfc/bcp/bcp47.txt (string)
style: CSS (string)
title: string
usemap: URL fragment including "#" (string)

COMMA LISTS:
accept: comma-delimited list of MIME types, "audio/*", "video/*", "image/*", or file extension starting with "."
srcset: comma-delimited list of image descriptors; see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/source#attr-srcset
coords: comma-delimited list of floats; count depends on shape attribute value
  rect → two x,y pairs: left, top, right, bottom
  circle → x,y,r
  poly → at least three x,y pairs

SPACE LISTS:
accept-charset: space- or comma-delimited list of character encodings
class: space-delimited list of string
for: space-delimited list of IDs
headers: space-delimited list of IDs
ping: space-delimited list of URLs
rel: space-delimited list of large enum
sandbox: space-delimited list of large enum
DONE WITH SPACE LISTS.

autocapitalize: enum {none, sentences, words, characters}
crossorigin: enum {anonymous, use-credentials}
decoding: enum {sync, async, auto}
dir: enum {ltr, rtl, auto}
enctype: enum {application/x-www-form-urlencoded, multipart/form-data, text/plain}
http-equiv: enum {content-security-policy, refresh}
kind: enum {subtitles, captions, descriptions, chapters, metadata}
method: enum {post, get, dialog}
preload: enum {none, metadata, auto}
referrerpolicy: enum {no-referrer, no-referrer-when-downgrade, origin, origin-when-cross-origin, same-origin, strict-origin, strict-origin-when-cross-origin, unsafe-url}
scope: enum {row, col, rowgroup, colgroup, auto}
shape: enum {rect, circle, poly, default}
target: enum {_self, _blank, _parent, _top}; also iframe name for <form>
wrap: enum {hard, soft}

autocomplete:
  form → enum {on, off}
  input → enum (many options)
  textarea → same as for input
datetime: an unholy union of fifty million date, time, and offset formats (string)
  ins → date with optional time
  del → date with optional time
  datetime → an unholy union of fifty million date, time, and offset formats, representing a date or time in one place or everywhere (string)
sizes: three different definitions for <link>, <img>, and <source> >:(
type:
  button → enum {submit, reset, button}
  input → large enum
  embed → MIME type (string)
  object → content type (string)
  script → "text/javascript", "module", or any other MIME type
  source → MIME type with an optional codecs parameter as in https://tools.ietf.org/html/rfc4281
  style → MIME type
value:
  button → string
  data → string
  input → string
  li → int
  meter → float
  option → string
  progress → float
  param → string