QUOTE:  “Tell visitors clearly that the page they’re looking for can’t be found. Use language that is friendly and inviting. Make sure your 404 page uses the same look and feel (including navigation) as the rest of your site. Consider adding links to your most popular articles or posts, as well as a link to your site’s home page. Think about providing a way for users to report a broken link. No matter how beautiful and useful your custom 404 page, you probably don’t want it to appear in Google search results. In order to prevent 404 pages from being indexed by Google and other search engines, make sure that your webserver returns an actual 404 HTTP status code when a missing page is requested.” Google, 2018

That content CAN be on links to your own content on other pages, but if you are really helping a user understand a topic – you should be LINKING OUT to other helpful resources e.g. other websites.A website that does not link out to ANY other website could be interpreted accurately to be at least, self-serving. I can’t think of a website that is the true end-point of the web.

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Creating high quality content takes a significant amount of at least one of the following: time, effort, expertise, and talent/skill. Content should be factually accurate, clearly written, and comprehensive. So, for example, if you describe your page as a recipe, provide a complete recipe that is easy to follow, rather than just a set of ingredients or a basic description of the dish.
Onsite, consider linking to your other pages by linking to pages within main content text. I usually only do this when it is relevant – often, I’ll link to relevant pages when the keyword is in the title elements of both pages. I don’t go in for auto-generating links at all. Google has penalised sites for using particular auto link plugins, for instance, so I avoid them.
As an Internet marketing strategy, SEO considers how search engines work, the computer-programmed algorithms that dictate search engine behavior, what people search for, the actual search terms or keywords typed into search engines, and which search engines are preferred by their targeted audience. SEO is performed because a website will receive more visitors from a search engine the higher the website ranks in the search engine results page (SERP). These visitors can then be converted into customers.[4]
You may not want certain pages of your site crawled because they might not be useful to users if found in a search engine's search results. If you do want to prevent search engines from crawling your pages, Google Search Console has a friendly robots.txt generator to help you create this file. Note that if your site uses subdomains and you wish to have certain pages not crawled on a particular subdomain, you'll have to create a separate robots.txt file for that subdomain. For more information on robots.txt, we suggest this Webmaster Help Center guide on using robots.txt files13.
Google knows who links to you, the “quality” of those links, and whom you link to. These – and other factors – help ultimately determine where a page on your site ranks. To make it more confusing – the page that ranks on your site might not be the page you want to rank, or even the page that determines your rankings for this term. Once Google has worked out your domain authority – sometimes it seems that the most relevant page on your site Google HAS NO ISSUE with will rank.
A lot of optimisation techniques that are in the short term effective at boosting a site’s position in Google are against Google’s guidelines. For example, many links that may have once promoted you to the top of Google, may, in fact, today be hurting your site and its ability to rank high in Google. Keyword stuffing might be holding your page back. You must be smart, and cautious, when it comes to building links to your site in a manner that Google *hopefully* won’t have too much trouble with, in the FUTURE. Because they will punish you in the future.
Does this article have an excessive amount of ads that distract from or interfere with the main content? (OPTIMISE FOR SATISFACTION FIRST – CONVERSION SECOND – do not let the conversion get in the way of satisfying the INTENT of the page. For example – if you rank with INFORMATIONAL CONTENT with a purpose to SERVE those visitors – the visitor should land on your destination page and not be deviated from the PURPOSE of the page – and that was informational, in this example – to educate. SO – educate first – beg for social shares on those articles – and leave the conversion on Merit and slightly more subtle influences rather than massive banners or whatever that annoy users). We KNOW ads (OR DISTRACTING CALL TO ACTIONS) convert well at the top of articles – but Google says it is sometimes a bad user experience. You run the risk of Google screwing with your rankings as you optimise for conversion so be careful and keep everything simple and obvious.
Critics will point out the higher the cost of expert SEO, the more cost-effective Adwords becomes, but Adwords will only get more expensive, too. At some point, if you want to compete online, your going to HAVE to build a quality website, with a unique offering to satisfy returning visitors – the sooner you start, the sooner you’ll start to see results.
A satisfying UX is can help your rankings, with second-order factors taken into consideration. A poor UX can seriously impact your human-reviewed rating, at least. Google’s punishing algorithms probably class pages as something akin to a poor UX if they meet certain detectable criteria e.g. lack of reputation or old-school SEO stuff like keyword stuffing a site.
Link building is not JUST a numbers game, though. One link from a “trusted authority” site in Google could be all you need to rank high in your niche. Of course, the more “trusted” links you attract, the more Google will trust your site. It is evident you need MULTIPLE trusted links from MULTIPLE trusted websites to get the most from Google in 2019.

QUOTE: “To summarize, a lack of helpful SC may be a reason for a Low quality rating, depending on the purpose of the page and the type of website. We have different standards for small websites which exist to serve their communities versus large websites with a large volume of webpages and content. For some types of “webpages,” such as PDFs and JPEG files, we expect no SC at all.” Google Search Quality Evaluator Guidelines 2017

If you are using Responsive Web Design, use meta name="viewport" tag to tell the browser how to adjust the content. If you use Dynamic Serving, use the Vary HTTP header to signal your changes depending on the user-agent. If you are using separate URLs, signal the relationship between two URLs by tag with rel="canonical" and rel="alternate" elements.
OBSERVATION – You can have the content and the links – but if your site falls short on even a single user satisfaction signal (even if it is picked up by the algorithm, and not a human reviewer) then your rankings for particular terms could collapse – OR – rankings can be held back – IF Google thinks your organisation, with its resources, or ‘reputation, should be delivering a better user experience to users.
Search engine optimization (SEO) is often about making small modifications to parts of your website. When viewed individually, these changes might seem like incremental improvements, but when combined with other optimizations, they could have a noticeable impact on your site's user experience and performance in organic search results. You're likely already familiar with many of the topics in this guide, because they're essential ingredients for any web page, but you may not be making the most out of them.