HTML

There is a doctype declaration
<!DOCTYPE html>
The html tag has a "lang" attribute
<html lang="en">
The charset is defined
<meta charset="utf-8">
The style and script tags do not have the "type" attribute.
<script src="..." ></script>
All tags and attributes are lowercase
<div> not <DIV>
Attribute values are quoted with double quotes
<div id="sidebar">...</div>

The document uses consistent indentation of child elements

        
        <div>
            <span>Hello!</span>
        </div>

Each open tag has a corresponding close tag (if one is required)
<span>Hello, world.</span>
Tags that are void do not have a close tag nor a trailing slash
(area, base, br, col, command, embed, hr, img, input, keygen, link, meta, param, source, track, wbr)
<hr> not <hr />
All images have alt attributes, except those that are used only for visual styling
<img src="mydog.jpg" alt="my dog Sparky">
Forced line breaks are used sparingly
<p>In other words, not this:</p><br><br><br>
Tables are only used for tabular data
<div>Use divs and CSS instead!</div>
The document does not use any deprecated tags or attributes
Not this! <body bgcolor="#eee">
The document uses inline styles sparingly or not at all
Not this! <body style="background-color:#eee">

CSS

Class and ID values are semantic, not presentational
#login { ... } /* versus #blue-border */
Class and ID values use either camelCase or hyphens for multiple words (and the choice is consistent)
#searchButton { ... } #search-button { ... }
The stylesheet uses CSS shorthand styles when possible
padding: 10px 5px 2px 5px;
Zero values do not have units specified
margin: 0; /* not 0px */
Colors use 3 character hexadecimal short-hand when possible
color:#222 /* not #222222 */
Every declaration ends with a semi-colon, even the last in the block
p { color:#222; font-size:1em; }
The stylesheet uses single quotes in property values
input[type='text'] { font-weight:bold; }
The stylesheet uses comments to group styles logically
/* Footer */ #footer { ... }
All style declarations are lowercase
p { color: #eee } not p { color:#EEE }
None of the CSS selectors use more than one ID
#header .search #quicksearch { ... } /* not good */

JavaScript/jQuery

The code uses proper white space and indentation

        
    $(document).ready(function() {
    $('#test').click(function() {
    console.log('this is not very readable');
    });
    });

        
    $(document).ready(function() {
        $('#test').click(function() {
            console.log('this is better');
        });
    });

Each statement terminates in a semi-colon

        
    function alertMe() {
        alert('hello')    //missing semi-colon
    }

There is only one statement per line

        
    var index = 0; index++; alert(index); // avoid

    //this is more readable for anything but the simplest statements
    var index = 0;
    index++;
    alert(index);

All variables are declared with var
var index = 0;
The code avoids the eval() statement
var userInfo = eval(feed); //no!
The code uses single quotes
var name = 'Bob';

The for... in loop is only used to iterate through properties of objects (not arrays)

        
    for (var prop in myObj) {
        //do something with myObj[prop]
    }

Functions and unclear statements are documented with code comments
var zip = elements[1]; //zip code is returned from API in position 1
Variable names are camelCase and are noun phrases
var firstName = "George";
Function names are lowercase and are verb phrases
function getName() { return firstName; }
Class names are SentenceCase and are noun phrases
function Car() { ... }
The code avoids redundant checks for boolean values
if (y != null && y != '') { ... } if (y) { ... } //does the same thing and is much shorter
The strict operator ("triple equals") is used when making comparisons
if (name === 'Bob') { ... } //not ==

The code within a conditional or iterator block (if, else, for, while etc.) is surrounded by braces

        
    for (var i = 0; i < arr.length; i++) 
        console.log(arr[i]); //this technically will not cause an error but should be avoided

The jQuery code makes use of function chaining where appropriate

        
    $('a.selected')
        .attr('id', 'searchLink')
        .css('color', '#fff')
        .click(function() {
            ...
        });

No line of code is more than 80 characters
// for your reference, this sentence demonstrates what 80 characters looks like