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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.

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Simply speaking, dofollow links are links missing the rel="nofollow" attribute. Such links are followed by search engines and pass PageRank (please note that links can also be restricted from following in bulk via the nofollow <meta> tag).

While there is nothing wrong with linking to other sites via dofollow links, if you link extensively to irrelevant or low-quality sites, search engines may conclude your site sells links or participates in other link schemes, and it can get penalized.

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While broken images on the website don't influence its search engine rankings directly, they definitely deserve being fixed for two reasons.

First and foremost, broken images are a crucial factor for user experience and may result in visitors bouncing away from the site without completing their goals.

And second, missing images may impede the site's crawling and indexation, wasting its crawl budget and making it hard for search engine bots to crawl some of the site's important content.

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While search engines can't read text off images, alt attributes (also known as "alternative attributes") help the former understand what your images portray.

The best practice is to create an alt text for each image, using your keywords in it when possible< to help search engines better understand your pages' content and hopefully rank your site higher in search results.

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Naturally, there's a direct correlation between the size of the page and its loading speed, which, in turn, is one of the numerous ranking factors. Basically, heavy pages load longer. That's why the general rule of thumb is to keep your page size up to 3MB. Of course, it's not always possible. For example, if you have an e-commerce website with a large number of images, you can push this up to more MBs, but this can significantly impact page loading time for users with a slow connection speed.

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URLs that contain dynamic characters like "?", "_" and parameters are not user-friendly because they are not descriptive and are harder to memorize. To increase your pages' chances to rank, it's best to setup URLs so that they would be descriptive and include keywords, not numbers or parameters. As Google Webmaster Guidelines state, "URLs should be clean coded for best practice, and not contain dynamic characters."

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URLs shorter than 115 characters are easier to read by end users and search engines, and will work to keep the website user-friendly.

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Broken outgoing links can be a bad quality signal to search engines and users. If a site has many broken links, they conclude that it has not been updated for some time. As a result, the site's rankings may be downgraded.

Although 1-2 broken links won't cause a Google penalty, try to regularly check your website, fix broken links (if any), and make sure their number doesn't go up. Besides, users will like your website more if it doesn't show them broken links pointing to non-existing pages.

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According to Matt Cutts (former head of Google's Webspam team), "...there's still a good reason to recommend keeping to under a hundred links or so: the user experience. If you're showing well over 100 links per page, you could be overwhelming your users and giving them a bad experience. A page might look good to you until you put on your "user hat" and see what it looks like to a new visitor." Although Google keeps talking about users experience, too many links on a page can also hurt your rankings. So the rule is simple: the fewer links on a page, the fewer problems with its rankings. So try to stick to the best practices and keep the number of outgoing links (internal and external) up to 100.

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If a page doesn't have a title, or the title tag is empty (i.e. it just looks like this in the code: <title></title>), Google and other search engines will decide for themselves which text to show as your page title in their SERP snippets. Thus, you'll have no control what people see on Google when they find your page.

Therefore, every time you are creating a webpage, don't forget to add a meaningful title that would also be attractive to users.

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A page title is often treated as the most important on-page element. It is a strong relevancy signal for search engines, because it tells them what the page is really about. It is, of course, important that title includes your most relevant keyword. But more to that, every page should have a unique title to ensure that search engines have no trouble in determining which of the website pages is relevant for a query. Pages with duplicate titles have fewer chances to rank high. Even more, if your site has pages with duplicate titles, this may negatively influence other pages' rankings, too.

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Every page should have a unique, keyword-rich title. At the same time, you should try to keep title tags concise. Titles that are longer than 70 characters get truncated by search engines and will look unappealing in search results. Even if your pages rank on page 1 in search engines, yet their titles are shortened or incomplete, they won't attract as many clicks as they would have driven otherwise.

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Although meta descriptions don't have direct influence on rankings, they are still important while they form the snippet people see in search results. Therefore, it should "sell" the webpage to the searcher and encourage them to click through.

If the meta description is empty, search engines will decide for themselves what to include into a snippet.

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According to Matt Cutts, it is better to have unique meta descriptions and even no meta descriptions at all, than to show duplicate meta descriptions across your pages. Hence, make sure that your top-important pages have unique and optimized descriptions.

About this SEO Factor:

Although meta descriptions don't have direct effect on rankings, they are still important while they form the snippet people see in search results. Therefore, descriptions should "sell" the webpage to the searchers and encourage them to click through. If the meta description is too long, it'll get cut by the search engine and may look unappealing to users.

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If you have a multi-language website with different regional versions of a page, there is good way to tell search engines about these localized variations by using hreflang elements.

In this section you can review the list of all hreflang values that have been found on pages of the project domain. You may also check Google guidelines (https://support.google.com/webmasters/answer/189077) for multi-language websites about how to use hreflang elements properly.

About this SEO Factor:

This factor checks and verifies hreflang elements that have been used on the project domain pages. For every page WebSite Auditor provides detailed report on the incorrect usage of hreflang elements and their values.

About this SEO Factor:

This factor analyses and verifies values of all hreflang elements found on your pages. One of the most common mistakes is to use incorrect language-country values for hreflang annotations, for example: 'en-UK' instead of 'en-GB'.
All hreflang values should comply with ISO 639-1 standards (https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) if only language versions of a page are specified or with ISO 3166-1 Alpha 2 (https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2) if both language and country versions are indicated. You can also use the "x-default" value for pages that do not have other better suited language/region versions.

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The factor validates all URLs that are used in hreflang attirbutes on your website. Make sure to use fully-qualified URLs for each language/region version of a page including the “http://” or “https://” protocol. It is not allowed to use relative URLs for hreflang attributes.

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Each language version of a page should have hreflang attributes that point to the page itself and also to other language/region versions of a page. It is also required that all language/region versions should point to each other in their hreflang attributes.

Example: there is a page in English(en) with 2 other language variants - Russian(ru) and German(de). The head section of the English version (en) should contain 3 hreflang elements: one pointing to the English version itself and 2 others pointing to the Russian(ru) and German(de) pages. Both German(de) and Russian(ru) variants should also have 3 hreflang elements that point back to the English version, to each other and also list themselves in hreflang attributes. Otherwise, if pages do not point back to each other their hreflang attributes will be ignored by search engines.

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This factor checks for conflicting hreflang values: a page language variant should have only one hreflang language attribute assigned (ex.: a page should not have both "en" and "de" hreflang values). Although, it is possible to assign the same language but different regions: en-US and en-GB.

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This factor lists all pages that have hreflang attributes but at the same time use a canonical element which points to some other page. Such a combination of elements can confuse search engines because it suggests different URLs for indexation. The best practice is to either remove the canonical tag or to link the canonical element to the page itself.

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This factor checks for pages that use hreflang elements without the "x-default" value. Using "x-default" is not obligatory but is a good way to tell search engines which page version it should use for languages and regions that have not been defined through your hregflang attributes.

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This factor analyzes all pages that use language-region hreflang attributes and verifies that all these pages also have the "x-default" value and provide a generic URL for geographically unspecified users of the same language. Example: a page may have specific variants for english speaking users from Canada(en-Ca) and Australia(en-Au) but you also need to specify a generic URL that will be used for English-speaking users from other countries.