Ideally, your title tag should contain between 10 and 70 characters (spaces included).
Make sure your title is explicit and contains your most important keywords.
Be sure that each page has a unique title.

Meta descriptions contains between 70 and 160 characters (spaces included).
It allow you to influence how your web pages are described and displayed in search results.
Ensure that all of your web pages have a unique meta description that is explicit and contains your most important keywords (these appear in bold when they match part or all of the user's search query).
A good meta description acts as an organic advertisement, so use enticing messaging with a clear call to action to maximize click-through rate.

Gzip is a method of compressing files (making them smaller) for faster network transfers.
It allows to reduce the size of web pages and any other typical web files to about 30% or less of its original size before it transfer.

Redirecting requests from a non-preferred domain is important because search engines consider URLs with and without "www" as two different websites.

A sitemap lists URLs that are available for crawling and can include additional information like your site's latest updates, frequency of changes and importance of the URLs. This allows search engines to crawl the site more intelligently.
We recommend that you generate an XML sitemap for your website and submit it to both Google Search Console and Bing Webmaster Tools. It is also good practice to specify your sitemap's location in your robots.txt file.

A robots.txt file allows you to restrict the access of search engine robots that crawl the web and it can prevent these robots from accessing specific directories and pages. It also specifies where the XML sitemap file is located.
You can check for errors in your robots.txt file using Google Search Console (formerly Webmaster Tools) by selecting 'Robots.txt Tester' under 'Crawl'. This also allows you to test individual pages to make sure that Googlebot has the appropriate access.

Your site's URLs contain unnecessary elements that make them look complicated.
A URL must be easy to read and remember for users. Search engines need URLs to be clean and include your page's most important keywords.
Clean URLs are also useful when shared on social media as they explain the page's content.

Great, you are not using ?underscores (these_are_underscores) in your URLs.
While Google treats hyphens as word separators, it does not for underscores.

Embedded Objects such as Flash. It should only be used for specific enhancements.
Although Flash content often looks nicer, it cannot be properly indexed by search engines.
Avoid full Flash websites to maximize SEO.

Frames can cause problems on your web page because search engines will not crawl or index the content within them.
Avoid frames whenever possible and use a NoFrames tag if you must use them.

Embedded Objects such as Flash, Silverlight or Java. It should only be used for specific enhancements.
But avoid using Embedded Objects, so your content can be accessed on all devices.

Keep your URLs short and avoid long domain names when possible.
A descriptive URL is better recognized by search engines.
A user should be able to look at the address bar and make an accurate guess about the content of the page before reaching it (e.g., http://www.mysite.com/en/products).

When a visitor encounters a 404 File Not Found error on your site, you're on the verge of losing the visitor that you've worked so hard to obtain through the search engines and third party links.
Creating your custom 404 error page allows you to minimize the number of visitors lost that way.

Two of the main reasons for an increase in page size are images and JavaScript files.
Page size affects the speed of your website; try to keep your page size below 2 Mb.
Tip: Use images with a small size and optimize their download with gzip.

Site speed is an important factor for ranking high in Google search results and enriching the user experience.
Resources: Check out Google's developer tutorials for tips on how to to make your website run faster.

Make sure your declared language is the same as the language detected by Google
Also, define the language of the content in each page's HTML code.

We don't recommend adding plain text/linked email addresses to your webpages.
As malicious bots scrape the web in search of email addresses to spam. Instead, consider using a contact form.

Safe Browsing to identify unsafe websites and notify users and webmasters so they can protect themselves from harm.

Web analytics let you measure visitor activity on your website.
You should have at least one analytics tool installed, but It can also be good to install a second in order to cross-check the data.

The Doctype is used to instruct web browsers about the document type being used.
For example, what version of HTML the page is written in.
Declaring a doctype helps web browsers to render content correctly.

Specifying language/character encoding can prevent problems with the rendering of special characters.