QUOTE: “Many SEOs and other agencies and consultants provide useful services for website owners, including: Review of your site content or structure – Technical advice on website development: for example, hosting, redirects, error pages, use of JavaScript – Content development – Management of online business development campaigns – Keyword research – SEO training – Expertise in specific markets and geographies.” Google Webmaster Guidelines, 2020
QUOTE: “Supplementary Content contributes to a good user experience on the page, but does not directly help the page achieve it purpose. SC is controlled by webmasters and is an important part of the user experience. One common type of SC is navigation links that allow users to visit other parts of the website. Note that in some cases, content behind tabs may be considered part of the SC of the page. Sometimes the easiest way to identify SC is to look for the parts of the page that are not MC or Ads. ” Google Search Quality Evaluator Guidelines 2019

Naturally, business owners want to rank for lots of keywords in organic listings with their website. The challenge for webmasters and SEO is that Google doesn’t want business owners to rank for lots of keywords using auto-generated content especially when that produces A LOT of pages on a website using (for instance) a list of keyword variations page-to-page.
Let's say, for example, you're researching the keyword "how to start a blog" for an article you want to create. "Blog" can mean a blog post or the blog website itself, and what a searcher's intent is behind that keyword will influence the direction of your article. Does the searcher want to learn how to start an individual blog post? Or do they want to know how to actually launch a website domain for the purposes of blogging? If your content strategy is only targeting people interested in the latter, you'll need to make sure of the keyword's intent before committing to it.
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.
QUOTE: “Returning a code other than 404 or 410 for a non-existent page (or redirecting users to another page, such as the homepage, instead of returning a 404) 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 (also, you probably don’t want your site to rank well for the search query” GOOGLE
KISS does not mean boring web pages. You can create stunning sites with smashing graphics – but you should build these sites using simple techniques – HTML & CSS, for instance. If you are new to web design, avoid things like Flash and JavaScript, especially for elements like scrolling news tickers, etc. These elements work fine for TV – but only cause problems for website visitors.
Being ‘relevant’ comes down to keywords & key phrases – in domain names, URLs, Title Elements, the number of times they are repeated in text on the page, text in image alt tags, rich markup and importantly in keyword links to the page in question. If you are relying on manipulating hidden elements on a page to do well in Google, you’ll probably trigger spam filters. If it is ‘hidden’ in on-page elements – beware relying on it too much to improve your rankings.

QUOTE: “You heard right there is no duplicate content penalty… I would avoid talking about duplicate content penalties for spam purposes because then it’s not about duplicate content then it’s about generating in a very often in an automated way content that is not so much duplicated as kind of fast and scraped from multiple places and then potentially probably monetized in some way or another and it just doesn’t serve any purpose other than to gain traffic redirect traffic maybe make some money for the person who created it this is not about content that gets really sort of created for any kind of reason other than just to be there so I person I don’t think about it as duplicate content there is just this disparity it’s the same thing as saying that maybe we created gibberish is some sort of a duplicate content of words but otherwise also exists like I I would really separate those two issues because the things that Aamon led in with URLs generated on WordPress and the things they were going to discuss today later maybe about e-commerce sites and so on I wouldn’t want people to confuse those two.”  Andrey Lipattsev, Google 2016

One common scam is the creation of "shadow" domains that funnel users to a site by using deceptive redirects. These shadow domains often will be owned by the SEO who claims to be working on a client's behalf. However, if the relationship sours, the SEO may point the domain to a different site, or even to a competitor's domain. If that happens, the client has paid to develop a competing site owned entirely by the SEO.