In addition to processing the text content on your web pages, Google will also try to figure out what your images are about as well. Alt Text is a short description that you can customize for each image to let Google know what the image is about. Setting short, descriptive Alt Texts through our site builder will help Google better associate your web pages with the search terms you're trying to target.
While that theory is sound (when focused on a single page, when the intent is to deliver utility content to a Google user) using old school SEO techniques on especially a large site spread out across many pages seems to amplify site quality problems, after recent algorithm changes, and so this type of optimisation without keeping an eye on overall site quality is self-defeating in the long run.
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.

If you link out to irrelevant sites, Google may ignore the page, too – but again, it depends on the site in question. Who you link to, or HOW you link to, REALLY DOES MATTER – I expect Google to use your linking practices as a potential means by which to classify your site. Affiliate sites, for example, don’t do well in Google these days without some good quality backlinks and higher quality pages.

However, we do expect websites of large companies and organizations to put a great deal of effort into creating a good user experience on their website, including having helpful SC. For large websites, SC may be one of the primary ways that users explore the website and find MC, and a lack of helpful SC on large websites with a lot of content may be a reason for a Low rating.
Search engines may penalize sites they discover using black or grey hat methods, either by reducing their rankings or eliminating their listings from their databases altogether. Such penalties can be applied either automatically by the search engines' algorithms, or by a manual site review. One example was the February 2006 Google removal of both BMW Germany and Ricoh Germany for use of deceptive practices.[54] Both companies, however, quickly apologized, fixed the offending pages, and were restored to Google's search engine results page.[55]

QUOTE: “Cleaning up these kinds of link issue can take considerable time to be reflected by our algorithms (we don’t have a specific time in mind, but the mentioned 6-12 months is probably on the safe side). In general, you won’t see a jump up in rankings afterwards because our algorithms attempt to ignore the links already, but it makes it easier for us to trust the site later on.” John Mueller, Google, 2018
While Google never sells better ranking in our search results, several other search engines combine pay-per-click or pay-for-inclusion results with their regular web search results. Some SEOs will promise to rank you highly in search engines, but place you in the advertising section rather than in the search results. A few SEOs will even change their bid prices in real time to create the illusion that they "control" other search engines and can place themselves in the slot of their choice. This scam doesn't work with Google because our advertising is clearly labeled and separated from our search results, but be sure to ask any SEO you're considering which fees go toward permanent inclusion and which apply toward temporary advertising.
QUOTE: “The score is determined from quantities indicating user actions of seeking out and preferring particular sites and the resources found in particular sites. *****A site quality score for a particular site**** can be determined by computing a ratio of a numerator that represents user interest in the site as reflected in user queries directed to the site and a denominator that represents user interest in the resources found in the site as responses to queries of all kinds The site quality score for a site can be used as a signal to rank resources, or to rank search results that identify resources, that are found in one site relative to resources found in another site.” Navneet Panda, Google Patent
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.
SEO may generate an adequate return on investment. However, search engines are not paid for organic search traffic, their algorithms change, and there are no guarantees of continued referrals. Due to this lack of guarantees and certainty, a business that relies heavily on search engine traffic can suffer major losses if the search engines stop sending visitors.[61] Search engines can change their algorithms, impacting a website's placement, possibly resulting in a serious loss of traffic. According to Google's CEO, Eric Schmidt, in 2010, Google made over 500 algorithm changes – almost 1.5 per day.[62] It is considered a wise business practice for website operators to liberate themselves from dependence on search engine traffic.[63] In addition to accessibility in terms of web crawlers (addressed above), user web accessibility has become increasingly important for SEO.
Black hat SEO attempts to improve rankings in ways that are disapproved of by the search engines, or involve deception. One black hat technique uses hidden text, either as text colored similar to the background, in an invisible div, or positioned off screen. Another method gives a different page depending on whether the page is being requested by a human visitor or a search engine, a technique known as cloaking. Another category sometimes used is grey hat SEO. This is in between black hat and white hat approaches, where the methods employed avoid the site being penalized but do not act in producing the best content for users. Grey hat SEO is entirely focused on improving search engine rankings.
We expect advertisements to be visible. However, you should not let the advertisements distract users or prevent them from consuming the site content. For example, advertisements, supplement contents, or interstitial pages (pages displayed before or after the content you are expecting) that make it difficult to use the website. Learn more about this topic.38

“Doorways are sites or pages created to rank highly for specific search queries. They are bad for users because they can lead to multiple similar pages in user search results, where each result ends up taking the user to essentially the same destination. They can also lead users to intermediate pages that are not as useful as the final destination.
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]
When using the Keyword Explorer, Ahrefs will also produce the "parent topic" of the keyword you looked up, as you can see in the screenshot above, underneath the Keyword Difficulty meter. A keyword's parent topic is a broader keyword with higher search volume than your intended keyword, but likely has the same audience and ranking potential -- giving you more a valuable SEO opportunity when optimizing a particular blog post or webpage.
Users will occasionally come to a page that doesn't exist on your site, either by following a broken link or typing in the wrong URL. Having a custom 404 page30 that kindly guides users back to a working page on your site can greatly improve a user's experience. Your 404 page should probably have a link back to your root page and could also provide links to popular or related content on your site. You can use Google Search Console to find the sources of URLs causing "not found" errors31.
Another reason is that if you're using an image as a link, the alt text for that image will be treated similarly to the anchor text of a text link. However, we don't recommend using too many images for links in your site's navigation when text links could serve the same purpose. Lastly, optimizing your image filenames and alt text makes it easier for image search projects like Google Image Search to better understand your images.
Write a description that would both inform and interest users if they saw your description meta tag as a snippet in a search result. While there's no minimal or maximal length for the text in a description meta tag, we recommend making sure that it's long enough to be fully shown in Search (note that users may see different sized snippets depending on how and where they search), and contains all the relevant information users would need to determine whether the page will be useful and relevant to them.