From http://www.w3schools.com/html/html5_intro.asp :

Even though everything says that utf-8 is the "default" character set,
Firefox complains unless pages explicitly have the meta line:
  <meta charset="utf-8">

Misc
    Cookies -> Local storage
Elements:  http://www.w3schools.com/html/html5_new_elements.asp
    Convention is LC tag and element names.
    datalist is just a type-ahead lookup for text fields.
        The <option>s within datalist are very different from <select><option>s.
            No separate value and label.
            <select><option>s must have text content for the label.
            <datalist><option>s have no text content and do not close.
    8 New semantic block elements (to construct a document outline):
        header, section, footer, aside, nav, main, article, figure
        [What about details, figcaption, mark, summary, time?]
          menuitem, details, summary, main no good in IE +.
        (For old browsers, need CSS so interpreted as block instead of inline,
        BUT since IE < 9 does not allow styling unknown els, need to use full
        work-around "the shive").
            section should contain a heading, but can nest arbitrarily
            article should be self-contained, but can nest arbitrarily
            captioned image = <figure><img/><figcaption/></figure>
                <figcaption> before <img> to position it above; and vice versa.
            (page/head) title is required
            new <header> is within body
    <wbr> not supported in IE.
    New form control attributes like number, date, time, calendar, and range.
    New graphic elements: <svg> and <canvas>.
      (0,0) is at top left corner.
        canvas is for JavaScript drawing.  Poor for text drawing but can do.
          High perf.  Can be saved to .png or .jpg.
          Draw with context: canvEl.getContext("2d");
          GOTCHA!  Use width and height attrs, NOT CSS, to size the canvas!
        svg is for XML drawing.  The svg objects are available to JS.
          vectors are located by cx/cy of their centers.
          Color with either stroke and fill vector attrs or vector styles.
          Styles may be set inline or with regular CSS selectors.
    New multimedia elements: <audio>, <video>, <embed> (and nested elements)
        Do specify size for video panel (by attr, not CSS).
        Apparently just width works.
    New custom elements allowed by syntax
        (still inline by default; you will normally want to add CSS).
    Meter (guage), display-only, not supported in IE.
Input elements:  13 new input types; 16 new attrs.
APIs:
    HTML Geolocation  With file: URLs doesn't work with Chrome.  Works with FF.
        Error-handling callback doesn't seem to fire with FF.
    HTML Drag and Drop
        Default behavior is paste the transfer data into allowing reciever.
        Default allowing receivers are text editors, HTML form fields, browser
         URL window.
          <el draggable="true" ondragstart='
            event.dataTransfer.setData("text-plain", "x");'/>
          For URL, add a "text/uri-list" for same data before the "text-plain".
        To transfer app-specific data, use a custom data type.
        Custom receivers must ALLOW via 'event.preventDefault()' for both
          ondrop= and ondragover= handlers.  The ondrop handler receives data:
          ondrop='event.preventDefault(); d = event.dataTransfer.getData(typ);'
        event.dataTransfer.getData(type) is always a String (so need to convert)
    HTML Local Storage
        Just use localStorage and sessionStorage maps.
        Remove entries with *Storage.removeItem("key");
        With file: URLs, localStorage saves are tab-specific.
        LOCAL STORAGE NOT WORKING even across browser tabs, for me,
         even when using an HTTP server with a FQHN URL.
    HTML Application Cache
        A manifest text file with custom syntax, normal suffix ".appcache".
          Server must service it with media type text/cache-manifest for it to work.
        NOT only for offline browsing.
        Specified resources will only be downloaded when the *.appcache file
         changes, regardless if he resource has changed or not.
    HTML Web Workers
        Default closure object is not 'window', so have no access to window, document, parent,
        but the onmessage listener (function) that you define does have closure of window.
        The provided listener listens until you w.terminate().
        Worker code calls postMessage(messageTextData);
        Worker code must be in an external file.  Booted with:
            var worker = new Worker("file.js");
        Can't re-use the file until you:  worker = undefined;
    HTML SSE
        Not supported by IE.
        Designed extremely analogously to Web Workers.
        URL service must return text/event-stream content
          beginning with "data: " ending with "\n\n".
          Internal occurrences of "\ndata: " will be removed.
        Contrary to w3schools, must register like
            source.addEventListener("message|open|error"), function(event) {...
        Use source.close(), not source.terminate();
        Automatically reconnects to the service after disconnects.
        Can check e.origin (in addition to e.data).
        Server can specify event types, and client side can listen for
          specific event types.
Removed elements:
    acronym
    applet
    basefont
    big
    center
    dir
    font
    frame
    frameset
    noframes
    strike
    tt

Legacy browser support
    Blockify the 8 new semantic els.
    JS "document.createElement(x);" for all new and custom elements.

"the shiv" workaround required for IE < 9 due to inability to style unknown elements:
Must be placed inside <head> (in reality, BEFORE body is effectively in head).
<!--[if lt IE 9]>
  <script src="http://html5shiv.googlecode.com/svn/trunk/html5.js"></script>
<![endif]-->


image best practices
Always specify size.  Do specify size for video panel (by attr or CSS).
Always specify alt text (alt="...").

other best practices
spaces around equals signs is allowed (but ugly)
html and body not required (but preferred)
no tabs; indent with 2 spaces
omit <head> and leverage that pre-body == inside head

Type attrs unnecessary for includes
<link rel="stylesheet" href="styles.css">
<script src="myscript.js"></script>

Vanilla JavaScript bootstrapping:
  <script>
    window.onload = function() {
      ...
    };
  </script>

Multi-media
Video
    Supported Video formats:  MP4, Ogg, WebM,
      but only MP4 supported by all HTML5 browsers.
    No MPEG (though MP3 audio is), AVI, WMV, QuickTime, RealVideo support.
    Flash requires browser support.
    Mime-types for source elements:  video/mp4, video/webm, video/ogg
    src attr not well supported, so use source element(s).
    autoplay and controls apparently incompatible with one another.
    autoplay has no effect on portable devices
    Javascript has full playback control.
<video id="video1" width="854" poster="/tmp/gr_level3.png" controls>
  <!-- Uses first source supported and found -->
  <source src="/tmp/bbb-480p.mp4" type="video/mp4">
  <source src="mov_bbb.ogg" type="video/ogg">
  Your browser does not support HTML5 video.
</video>
Audio
    No support:  MIDI, RealAudio, WMA, AAC
    Supported:  WAV, Ogg, MP3, MP4

HTML and CSS Code Validation.  Enter your URL at
http://www.w3schools.com/website/web_validate.asp

Forms
Supported input types:  http://www.w3schools.com/tags/att_input_type.asp
(type still defaults to "text").
Input validation seems to always occur only upon submit.
<input pattern=...>
    Does not work with safari
    title attr value used for both tooltip + syntax error message
    Impossible to specify a modifier (like case-insensitive)
    Test only runs upon submit
    pattern must match entire string.  It's like Java String.match().
    type "number" renders with a spinner for Chrome and FF, not IE 11.
    "number" is for integers (negatives supported)
<input required>  Does not work with Safari
PORTABLE types (*=lacking only Safari):  email*, number, range, url* + all HTML4
  IE 11 displays range as text field.
Support problems for input types:
    type "datetime" deprecated (for "datetime-local")
    color
    date
    datetime
    datetime-local
    email only lacking in Safari
    month
    search
    tel
    time
    url only lacking in Safari
    week
Type "image" is just a submit button with an image.
New input attributes
    Most just allow for input-specific overrides of the form attributes.
    autocomplete
    novalidate
    autofocus (for one input)
    list for <input type="text"...  list-> <datalist>
        It remains a text list and allows arbitrary text entry.  Not a select!
    multiple for types email, file
    pattern
    height, width for type image
    pattern
    placeholder
    required
    min,max,step

readonly seems to have no effect on input types checkbox, radio, submit.
(disabled works great)

ID requirements relaxed:
an id must contain at least one character and may not contain any space characters.

To use a new tag name in IE, must run document.createElement("X") (just disregard the generated EL).
There is no custom element feature for HTML5
I think notes in here about IE limitations for range and number are wrong.

progress tag styling is difficult because it's so browser-dependent.
Default colors:
Chrome  blue over light gray
IE      dark cyan over gray
FF      green over lightish gray
To set a tooltip, do set title attr.
The body value is (IIRC) used for browsers that can't render the progress meter.
IE styles as completely unstyled div rectangles.
FF styles as slightly styled div rectangles.
Chrome styles fancy.  But to customize any styling you must disable the
  custom styling and go to plain rectangles first.