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4xx errors often point to a problem on a website. For example, if you have a broken link on a page, and visitors click it, they may see a 4xx error. It's important to regularly monitor and fix these errors, because they may have negative impact and lower your site's authority in users' eyes.

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5xx error messages are sent when the server has a problem or error. It's important to regularly monitor these errors and investigate their causes, because they may have negative impact and lower the site's authority in search engines' eyes.

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A custom 404 error page can help you keep users on the website. In a perfect world, it should inform users that the page they are looking for doesn't exist, and feature such elements as your HTML sitemap, the navigation bar and a search field. But more importantly, a 404 error page should return the 404 response code. This may sound obvious, but unfortunately it's rarely so.

According to Google Search Console:

"Returning a code other than 404 or 410 for a non-existent page... can be problematic. Firstly, it tells search engines that there's a real page at that URL. As a result, that URL may be crawled and its content indexed. Because of the time Googlebot spends on non-existent pages, your unique URLs may not be discovered as quickly or visited as frequently and your site's crawl coverage may be impacted. We recommend that you always return a 404 (Not found) or a 410 (Gone) response code in response to a request for a non-existing page."

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The robots.txt file is automatically crawled by robots when they arrive at your website. This file should contain commands for robots, such as which pages should or should not be indexed. If you want to disallow indexing of some content (for example, pages with private or duplicate content), just use an appropriate rule in the robots.txt file. For more information on such rules, check out http://www.robotstxt.org/robotstxt.html.

Please note that commands placed in the robots.txt file are more like suggestions rather than absolute rules for robots to follow. There's no guarantee that some robot will not check the content that you have disallowed.

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An XML sitemap should contain all of the website pages that you want to be indexed, and should be located on the website one directory structure away from the homepage (ex. http://www.site.com/sitemap.xml). In general, it serves to aid indexing. You should update it each time you add new pages to your website. Besides, the sitemap should follow particular syntax.

The sitemap allows you to set the priority of each page, telling search engines which pages they are supposed to crawl more often (i.e. they are more frequently updated). Learn how to create an .xml sitemap at http://www.sitemaps.org/.

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A resource can be restricted from indexing in several ways:

• in the robots.txt file;
• by Noindex X-Robots tag;
• by Noindex Meta tag.

Each of these is a line of HTML code that says how crawlers should handle certain resources on the site. Specifically, the tag tells crawlers if they are not allowed to index the page or resource, follow its links, and/or archive its contents. So make sure that all your unique and useful content is available for indexing.

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Usually websites are available with and without "www" in the domain name. Merging both URLs will help you prevent search engines from indexing two versions of a website.

Although the indexing of both versions won't cause a penalty, setting one of them as a priority is a best practice, in part because it helps funnel the SEO value from links to one common version. You can look up or change your current primary version in the .htaccess file. Also, it is recommended to set the preferred domain in Google Search Console.

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Using secure encryption is highly recommended for many websites (for instance, those taking transactions and collecting sensitive user information.) However, in many cases, webmasters face technical issues when installing SSL certificates and setting up the HTTP/HTTPS versions of the website.

In case you're using an invalid SSL certificate (ex. untrusted or expired one), most Web browsers will prevent users from visiting your site by showing them an "insecure connection" notification.

If the HTTP and HTTPS versions of your website are not set properly, both of them can get indexed by search engines and cause duplicate content issues that may undermine your website rankings.

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302 redirects are temporary, so they don't pass any link juice. If you use them instead of 301s, search engines may continue to index the old URLs, and disregard the new ones as duplicates. Or they may divide the link popularity between the two versions, thus hurting search rankings. That's why it is not recommended to use 302 redirects if you are permanently moving a page or a website. Stick to a 301 redirect instead to preserve link juice and avoid duplicate content issues.

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301 redirects are permanent and are usually used to solve problems with duplicate content or to redirect certain URLs that are no longer necessary. The use of 301 redirects is absolutely legitimate, and it's good for SEO because a 301 redirect will funnel link juice from the old page to the new one. Just make sure you redirect old URLs to the most relevant pages.

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In certain cases, either due to bad .htaccess file setup or due to some deliberately taken measures, a page may end up with having two or more redirects. It is strongly recommended to avoid such redirect chains longer than 2 redirects since they may be the reason of multiple issues:

• There is a high risk that a page will not be indexed as Google bots do not follow more than 5 redirects.
• Too many redirects will slow down your page speed. Every new redirect may add up to several seconds to the page load time.
• High bounce rate: users are not willing to stay on a page that takes more than 3 seconds to load.

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Basically, Meta refresh may be seen as a violation of Google's Quality Guidelines and therefore is not recommended from the SEO point of view. As one of Google's representatives points out: "In general, we recommend not using meta-refresh type redirects, as this can cause confusion with users (and search engine crawlers, who might mistake that for an attempted redirect)... This is currently not causing any problems with regards to crawling, indexing, or ranking, but it would still be a good idea to remove that." So stick to the permanent 301 redirect instead.

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In most cases duplicate URLs are handled via 301 redirects. However sometimes, for example when the same product appears in two categories with two different URLs and both need to be live, you can specify which page should be considered a priority with the help of rel="canonical" tags. It should be correctly implemented within the <head> tag of the page and point to the main page version that you want to rank in search engines. Alternatively, if you can configure your server, you can indicate the canonical URL using rel="canonical" HTTP headers.

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Using secure encryption is highly recommended for many websites (for instance, those taking transactions and collecting sensitive user information.) However, in many cases, webmasters face technical issues when switching their websites from HTTP to HTTPS. And one of these issues is the so-called mixed content - e.i. when your secure HTTPS pages include insecure content served over HTTP.

If an HTTPS page includes content retrieved through regular, cleartext HTTP, this weakens the security of the entire page as the unencrypted content is accessible to sniffers and can be modified by man-in-the-middle attackers. For this reason, in many modern browsers, such content might get blocked from loading or load with an "insecure connection" warning.

According to Google, there are two types of mixed content:
The two types of mixed content are: active and passive. Passive mixed content refers to content that doesn't interact with the rest of the page, and thus a man-in-the-middle attack is restricted to what they can do if they intercept or change that content. Passive mixed content includes images, video, and audio content, along with other resources that cannot interact with the rest of the page. Active mixed content interacts with the page as a whole and allows an attacker to do almost anything with the page. Active mixed content includes scripts, stylesheets, iframes, flash resources, and other code that the browser can download and execute.

Well done! Your site's homepage is mobile-friendly.

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According to Google, the mobile-friendly algorithm affects mobile searches in all languages worldwide and has a significant impact in Google's search results. This algorithm works on a page-by-page basis - it is not about how mobile-friendly your pages are, it is simply are you mobile-friendly or not.

The algo is based on such criteria as small font sizes, tap targets/links, readable content, your viewpoint, etc.

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Having multiple canonical URLs specified for a page happens frequently in conjunction with SEO plugins that often insert a default rel="canonical" link, possibly unknown to the webmaster who installed the plugin. Double-checking the page's source code and your server's rel="canonical" HTTP headers configurations will help correct the issue.

In case of multiple rel="canonical" declarations, Google will likely ignore all the rel=canonical hints, so your effort to avoid duplicate content issues may go useless.

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Frames allow displaying more than one HTML document in the same browser window. As a result, text and hyperlinks (the most important signals for search engines) appear missing from such documents. If you use Frames, search engines will fail to properly index your valuable content.

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The validation is usually performed via the W3C Markup Validation Service. And although compliance with W3C standards is not obligatory and will not have direct SEO effect, bad code may be the cause of Google not indexing your important content properly. It's recommended fix your pages' broken code to avoid issues with search engine spiders.

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The validation is usually performed via the W3C Markup Validation Service (W3C stands for World Wide Web Consortium).

CSS styles are used to control the design and formatting of the page, and to separate styles from the structure, which ultimately makes the page load faster.

Errors in CSS may be not that important to search engines, but they can lead to your page being incorrectly displayed to visitors, which, in turn, may affect your conversion and bounce rates. So, make sure the page is displayed as intended across all browsers (including mobile ones) important to you.