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The best length of description should be in 70-320 characters.

This is a number of signs in page's text. Recommended length of the text is much than 500 sighns.

Content part (visible text) should be more then 10% of all html source code of your page.

Many websites serve users from around the world with content translated or targeted to users in a certain region.
Google uses the rel="alternate" hreflang="x" attributes to serve the correct language or regional URL in Search results.

Text sometimes exhibits case sensitivity; that is, words can differ in meaning, or can be distinguished in a computer search, based on differing use of capital and lowercase letters.

Injecting parameters are variables which can be added to the link to an site.

Check if your website is using HTTPS, a secure protocol for sending/receiving data over the Internet. Using HTTPS indicates that an additional encryption/authentication layer was added between client and server.
HTTPS should be used by any site that collects sensitive customer data such as credit card information.
Even for sites that do not collect such data, switching to https helps users by improving privacy and overall security. Google is increasingly using https as a positive ranking factor.

Hiding text or links in your content to manipulate Google’s search rankings can be seen as deceptive and is a violation of Google’s Webmaster Guidelines.

Test your site for potential URL canonicalization issues. Canonicalization describes how a site can use slightly different URLs for the same page
(e.g., if http://www.example.com and http://example.com displays the same page but do not resolve to the same URL).
If this happens, search engines may be unsure about which URL is the correct one to index.

The web site hosting server will typically generate a "404 Not Found" web page when a user attempts to follow a broken or dead link;
hence the 404 error is one of the most recognizable errors encountered on the World Wide Web.

We check your page under Google bot and verify number of scripts and backlinks. Total amount of backlinks should be the same for all user agents

We check your page under Yandex bot and verify number of scripts and backlinks. Total amount of backlinks should be the same for all user agents

How Google User Experience algorithm evalutes your mobile version.

This rule triggers when Sitechecker detects that text in the page is too small to be legible.

A viewport controls how a webpage is displayed on a mobile device. Without a viewport, mobile devices will render the page at a typical desktop screen width, scaled to fit the screen.
Setting a viewport gives control over the page's width and scaling on different devices.

This rule triggers when Sitechecker detects that the page content does not fit horizontally within the specified viewport size, thus forcing the user to pan horizontally to view all the content.

Plugins help the browser process special types of web content, such as Flash, Silverlight, and Java.
Most mobile devices do not support plugins, and plugins are a leading cause of hangs, crashes, and security incidents in browsers that provide support.

If the amount of data required exceeds the initial congestion window (typically 14.6kB compressed), it will require additional round trips between your server and the user’s browser.
For users on networks with high latencies such as mobile networks this can cause significant delays to page loading.

Checks if any external javascript files used in your page are minified. Minified files reduce page size and overall load time.

Checks if any CSS files used in your page are minified. Minified files reduce page size and overall load time.

Check if your website is using HTML compression. HTML compression plays an important role in improving website speed by finding similar strings within a text file and replacing them temporarily to reduce overall file size.

Redirects trigger an additional HTTP request-response cycle and delay page rendering. In the best case, each redirect will add a single roundtrip (HTTP request-response), and in the worst it may result in multiple additional roundtrips to perform the DNS lookup, TCP handshake, and TLS negotiation in addition to the additional HTTP request-response cycle. As a result, you should minimize use of redirects to improve site performance.