Meta keywords are keywords inside Meta tags. Meta keywords are not likely to be used for search engine ranking. the words of title and description can be used as meta keywords. it is a good idea for SEO other than search engine ranking.

Keyword usage is the using of your keywords inside Meta tags and contents of your website. Use keywords that describes your site properly for precise search engine result of your website.

The ideal page's ratio of text to HTML code must be lie between 20 to 60%. Because if it is come less than 20% it means you need to write more text in your web page while in case of more than 60% your page might be considered as spam.

Sitemap is a xml file which contain full list of your website urls. It is used to include directories of your websites for crawling and indexing for search engine and access for users. it can help search engine robots for indexing your website more fast and deeply. It is roughly an opposite of robots.txt You can create a sitemap.xml by various free and paid service or you can write it with proper way (read about how write a sitemap).

Also keep these things in mind:
1) Sitemap must be less than 10 MB (10,485,760 bytes) and can contain maximum 50,000 urls. if you have more uls than this create multiple sitemap files and use a sitemap index file.
2) Put your sitemap in website root directory and add the url of your sitemap in robots.txt.
3) sitemap.xml can be compressed using grip for faster loading.

Broken link: a broken link is an inaccessible link or url of a website. a higher rate of broken links have a negative effect on search engine ranking due to reduced link equity. it also has a bad impact on user experience. There are several reasons for broken link. All are listed below.
1) An incorrect link entered by you.
2) The destination website removed the linked web page given by you. (A common 404 error).
3) The destination website is irreversibly moved or not exists anymore. (Changing domain or site blocked or dysfunctional).
4) User may behind some firewall or alike software or security mechanism that is blocking the access to the destination website.
5) You have provided a link to a site that is blocked by firewall or alike software for outside access.
Learn more or Learn more

GZIP is a generic compressor that can be applied to any stream of bytes: under the hood it remembers some of the previously seen content and attempts to find and replace duplicate data fragments in an efficient way - for the curious, great low-level explanation of GZIP. However, in practice, GZIP performs best on text-based content, often achieving compression rates of as high as 70-90% for larger files, whereas running GZIP on assets that are already compressed via alternative algorithms (e.g. most image formats) yields little to no improvement. It is also recommended that, GZIP compressed size should be <=33 KB

Inline css is the css code reside in html page under html tags not in external .css file. Inline css increases the loading time of your webpage which is an important search engine ranking factor. So try not to use inline css.

Internal css is the css codes which resides on html page inside style tag. Internal css is increases loading time since no page caching is possible for internal css. Try to put your css code in external file.

If multiple domain name is registered under single ip address the search bots can label other sites as duplicates of one sites. This is ip canonicalization. Little bit like url canonicalizaion. To solve this use redirects.
Learn more

Canonical tags make your all urls those lead to a single address or webpage into a single url. Like :
<link rel="canonical" href="https://mywebsite.com/home" />
<link rel="canonical" href="https://www.mywebsite.com/home" />
Both refer to the link mywebsite.com/home. So all the different url with same content or page now comes under the link or url mywebsite.com/home. Which will boost up your search engine ranking by eliminating content duplication. Use canonical tag for all the same urls.
Learn more