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.

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.

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."

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.

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/.

A resource can be restricted from indexing in several ways:

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

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.

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.

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.

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.

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.