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

An SEO friendly link is roughly follows these rules. The url should contain dash as a separator, not to contain parameters and numbers and should be static urls.

To resolve this use these techniques.
1) Replace underscore or other separator by dash, clean url by deleting or replaceing number and parameters.
2) Marge your www and non www urls.
3) Do not use dynamic and related urls. Create an xml sitemap for proper indexing of search engine.
4) Block unfriendly and irrelevant links through robots.txt.
5) Endorse your canonical urls in canonical tag.
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

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

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.

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

Plain text email address is vulnerable to email scrapping agents. An email scrapping agent crawls your website and collects every Email address which written in plain text. So existence of plain text email address in your website can help spammers in email Harvesting. This could be a bad sign for search engine.

To fight this you can obfuscate your email addresses in several ways:
1) CSS pseudo classes.
2) Writing backward your email address.
3) Turn of display using css.
4) Obfuscate your email address using javascript.
5) Using wordpress and php (wordpress site only).
Learn more