Description is the full interpretation of your website content and features. Most often it is a short paragraph that describe what are features and information provided by the website to its visitors. You may consider it a advertising of your website. Although not important for search engine ranking but very important for hits or visits through search engine results. Description should be less than 150 character because search engine shows this length of paragraph on search result. And every page of website should contain an unique description to avoid description duplication. Description is the definition of your website for user experience so form it as complete but short and precise illustration of your website.

robots.txt is text file that reside on website root directory and contains the instruction for various robots (mainly search engine robots) for how to crawl and indexing your website for their webpage. robots.txt contains the search bots or others bots name, directory list allowed or disallowed to be indexing and crawling for bots, time delay for bots to crawl and indexing and even the sitemap url. A full access or a full restriction or customized access or restriction can be imposed through robots.txt.

robots.txt is very important for SEO. Your website directories will be crawled and indexed on search engine according to robots.txt instructions. So add a robots.txt file in your website root directory. Write it properly including your content enriched pages and other public pages and exclude any pages which contain sensitive information. Remember robots.txt instruction to restrict access to your sensitive information of your page is not formidable on web page security ground. So do not use it on security purpose.
Learn more

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

doc type is not SEO factor but it is checked for validating your web page. So set a doctype at your html page.
Learn more

Older HTML tags and attributes that have been superseded by other more functional or flexible alternatives (whether as HTML or as CSS ) are declared as deprecated in HTML4 by the W3C - the consortium that sets the HTML standards. Browsers should continue to support deprecated tags and attributes, but eventually these tags are likely to become obsolete and so future support cannot be guaranteed.

HTML page size is the one of the main factors of webpage loading time. It should be less than 100 KB according to google recommendation. Note that, this size not including external css, js or images files. So small page size less loading time.

To reduce your page size do this steps
1) Move all your css and js code to external file.
2) make sure your text content be on top of the page so that it can displayed before full page loading.
3) Reduce or compress all the image, flash media file etc. will be better if these files are less than 100 KB
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