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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.

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).

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.

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

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.

This is the number of pages that we have discovered on your website.
A low number can indicate that bots are unable to discover your webpages, which is a common cause of a bad site architecture & internal linking, or you're unknowingly preventing bots and search engines from crawling & indexing your pages.