The principle behind this page is you MUST KNOW what Google has stated very clearly
about what it wants to see on websites
​- how they are written,
-  whether they are desktop or mobile,
- for text, images and video

Google has a section where it keeps ALL its GUIDANCE for those designing, coding, writing and maintaining websites.
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What I'm doing with this page is showing you where that is and highlighting / quoting of what it says. (Google licences the content under the Creative Commons Attribution 4.0 License - which means it can be shared online)

It's down to you to read the guidelines!
GOOGLE GUIDELINES - THE SHORTCUT

Webmaster Guidelines
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I'm quoting these in full - they are well worth learning!
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Following the General guidelines will help Google find, index, and rank your site.
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We strongly encourage you to pay very close attention to the Quality guidelines, which outline some of the illicit practices that may lead to a site being removed entirely from the Google index or otherwise affected by an algorithmic or manual spam action. If a site has been affected by a spam action, it may no longer show up in results on Google.com or on any of Google's partner sites.

General Guidelines
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Three principles
  • Help Google find your pages
  • Help Google understand your pages
  • Help visitors use your pages
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The detailed versions of all the above can be found on the Webmaster Guidelines page and you can find a shorthand version in the summaries below
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Help Google find your pages​
  • Ensure that all pages on the site can be reached by a link from another findable page​
  • Provide a sitemap file with links that point to the important pages on your site
  • Limit the number of links on a page to a reasonable number
  • Prevent crawling of infinite spaces such as search result pages.
  • ​Ask Google to crawl your pages
Help Google understand your pages
  • Create a useful, information-rich site
  • Write pages that clearly and accurately describe your content.
  • Design your site to have a clear conceptual page hierarchy.
  • Make your site's important content visible by default
  • ​Make sure your pages include the words which people will use to find them
  • Use accurate and very specific titles to describe your page
  • Use accurate <alt> alt attributes for images that are descriptive, specific, and accurate
  • Allow all site assets that would significantly affect page rendering to be crawled
  • Allow search bots to crawl your site
  • Make a reasonable effort to ensure that advertisement links on your pages do not affect search engine rankings
  • use the blocked resources report in Search Console and the Fetch as Google and robots.txt Tester tools to see which page assets that Googlebot cannot crawl, or to debug directives in your robots.txt file, 
  • When using a content management system (for example, Wix or WordPress), make sure that it creates pages and links that search engines can crawl.
Help visitors use your pages
  • Use text rather than images for important information e.g. when you want to display
    • important names,
    • content, or
    • links.
    • ( for example - on this website, all links are embedded in text unless the image needs a source link)
    • Use the the alt attribute to include a few words of descriptive text if images are used in place of content, 
  • ALWAYS use clean and valid HTML and CSS. Make sure all links go to live web pages. (You'll need to recheck this over time as links grow old)
  • Make sure your pages load quickly. Optimize page loading times. Fast sites make users happy and improve the overall quality of the web (especially for those users with slow Internet connections). 
  • Make sure different devices (desktops, tablets, and smartphones.) and screen sizes can display your website.  
  • Test your site in as many browsers as possible - and make sure it works in all
  • Make sure your site is secure. You should secure your site's connections with HTTPS. Encrypting interactions between the user and your website is a good practice for communication on the web.
  • Make sure people who are visually impaired can access your pages e.g. test usability with a screen-reader.
Useful tools recommended by Google

Google recommends that you use tools such as the ones listed below for specific purposes

Validate your HTML code Link checker Test the performance of your page. Website design for all devices
  • Mobile-Friendly Test 
    • tests how well your site / pages work on mobile devices, and
    • you get feedback on what needs to be fixed.
Compatibility with different browsers Site security
Picture
Best Practices when implementing HTTPS | Secure your site with HTTPS | Google

Content - Specific Guidelines
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These links are copied direct from the Content Guidelines page. Do check if you want to make sure they are up to date.
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Quality Guidelines - THESE ARE REALLY IMPORTANT!
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Basic principles​

  • Make pages primarily for users, not for search engines.
  • Don't deceive your users.
  • Avoid tricks intended to improve search engine rankings. A good rule of thumb is whether you'd feel comfortable explaining what you've done to a website that competes with you, or to a Google employee. Another useful test is to ask, "Does this help my users? Would I do this if search engines didn't exist?"
  • Think about what makes your website unique, valuable, or engaging. Make your website stand out from others in your field.
  • Quality Content | Webmaster Guidelines - Documentation | Google (my bold)
AVOID The following techniques:
Google regards all the above techniques are very naughty. 
FOLLOW good practices:
  • Monitoring your site for hacking and removing hacked content as soon as it appears
  • Preventing and removing user-generated spam on your site 

I recommend you ALWAYS moderate all comments and contributions to a page.
​Never ever let anybody contribute content to a page without you reviewing and authorising it first
What happens if you violate a guideline

If your site violates one or more of these guidelines, then Google may take manual action against it.

​Once you have remedied the problem, you can submit your site for reconsideration.

Quality Content | Webmaster Guidelines - Documentation | Google

VIDEOS

Google also has various videos which explain