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

The best way is to use only one H1 tag per page.

Text indicating the name of the page.

H1 and Title must match each other

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

Meta tags are snippets of text that describe a page's content; the meta tags don't appear on the page itself, but only in the page's code.

The X-Robots-Tag can be used as an element of the HTTP header response for a given URL.

The submitted URL can be crawled and indexed by search engine bots. Make sure it really needs to be scanned and visible in SERP. In order to determine what pages need to be closed from crawling by search bots, read our guide at #robotsornoindex.

Check if your webpage is using the noindex meta tag. Usage of this tag instructs search engines not to show your page 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.

Duplicated slashes are variables which can be added to the link to an site.

HTTPS (Hypertext Transfer Protocol Secure) is an internet communication protocol that protects the integrity and confidentiality of data between the user's computer and the site.

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.

You can access your site in two ways: http://example.com or http://example.com/index.html(or .php). Only one way should be used. Both ways is incorrect.

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.

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

Minification refers to the process of removing unnecessary or redundant data without affecting how the resource is processed by the browser - e.g. code comments and formatting, removing unused code, using shorter variable and function names, and so on.

Server response time measures how long it takes to load the necessary HTML to begin rendering the page from your server, subtracting out the network latency between Google and your server.
There may be variance from one run to the next, but the differences should not be too large. In fact, highly variable server response time may indicate an underlying performance issue.

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.