Descriptive URLs: use a descriptive page url, which should reflect your targeted keyword
Subdomain or subfolder: subdomains are seen as separate domains
Hyphans: split words using hyphans
www or no-www: provide both domains, but set a prefered version in Google Webmaster Tools
Localisation: chookse a country-specific domain, for better local search results
HTTPS: Security is a top priority for Google

Meta Information

Title: each page shound have a unique speaking title (60 - 100 characters) <title>Website Title</title>
Description: each page should have a unique description (max. 160 characters) <meta name="description" content="">

Keywords

Single: every page shound have a single unique targeted keyword
Research: rank for keywords with high traffic and less competition
URL: keyword should appear in URL name
Title: keyword should appear in page title
Heading: keyword should appear in headings
Content: keyword should apear in ~3% of article length
Meta Tag: you can ommit the <meta name="keywords" content="">, search engines do not use this meta tag

Content

Content: Content matters the most in SEO
Headings: Clear structure H1 - H6 max. 70 characters long
Strong: use strong tag to highlight your targeted keyword
Unique: do not provide duplicated content, use unique content types
Length: article should be at least 300 words
Freshness: new content is important. Updating pages or regulary posting is recommended

Images

File name: use a short descriptive name
Alt tag: add an alt-tag this a description of the image (60 - 70 characters)
Dimentions: add the width="" and height="" attributes to the image
Responsive Images: serve the most optimized image corresponding to the window size
Size: keep the filesize as low as possible
Optimization: Optimize images by removing some meta information

Links

add meaningful title
Only add external links if you got a backlink to your site
add rel="nofollow" attribute to external links
add ~ 3 internal links to your content

Social link rank up your site
use social snippets OpenGraph, Facebook, Twitter
Authorship information

<link rel="author" href="https://plus.google.com/u/0/[GOOGLE+ ID]">

<a href="https://plus.google.com/u/0/[GOOGLE+ ID]?rel=author">Google</a>

social profiles

<span itemscope itemtype="http://schema.org/Organization">
  <link itemprop="url" href="http://www.your-company-site.com">
  <a itemprop="sameAs" href="http://www.facebook.com/your-company">Facebook</a>
  <a itemprop="sameAs" href="http://www.twitter.com/YourCompany">Twitter</a>
</span>

Sitemap

HTML sitemap: An HTML sitemap allows site visitors to easily navigate a website.
XML sitemap: Help search engines to index your pages
Image sitemap: Increase that your images can be found in Image Search results
Video sitemap: Make sure, search engines know about all the video content on your site
Mobile sitemap: For feature phones, you can create a mobile sitemap

Errors and access

provide 403 - Acced denied page
provide 404 - Page not found page
add a robots.txt file
Avoid FILE_NOT_FOUND errors

Performance

Performance and loading time matters
only serve concatenated and minified files
if possible no redirects
compress images (ImageOptim)

UX

User friendly site
clickable links should not be too small

Mobile

add viewport tag

 <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1.0">

create a mobile-friendly site (shown in search results)
Mobile-Friendly Test
AppLinks

Validation and accessibility

Write valid markup (HTML Validator CSS Validator)
use WAI-Aria tags
use RichSnippets

Search

Provide a custom search (schema.org/SearchAction, RichSnippet)

readme.md

Search Engine Optimization (SEO)

Table of Contents

URL

Meta Information

Keywords

Content

Images

Links

Social Media

Sitemap

Errors and access

Performance

UX

Mobile

Validation and accessibility

Search

Analytics

Tools