Title is the heading of the webpage. The sentence or string enclosed between HTML title tag () is the title of your website. Search engines are searching for the title of your website and display the title along with your website address on search results. The title is the most important element for both SEO and social sharing. The title should be less than 50 to 60 characters because search engine typically displays this length of string or sentence on search results. A good title can consist of the primary keyword, secondary keyword and the brand name. For example a fictitious gaming information providing sites title may be like "the future of gaming information is here". A webpage title should contain a proper glimpse of the website. As said, the title is an important element as an identification of your website for your users experience, SEO and as well social sharing. So you should have a nice and catching title.
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The description is the full interpretation of your website content and features. Most often it is a short paragraph that describes what the features are and what information are provided by the website to its visitors. You may consider it as an advertising of your website. Although this is not too important for search engine ranking, but it is very important for hits or visits through search engine results. The description should be less than 150 characters because search engine shows this length of paragraph on search results. And every page of the website should contain a unique description to avoid description duplication. The description is the definition of your website for user experience, so form it as complete, but short and precise as possible.

robots.txt is a text file that reside on the websites root directory and contains the instruction for various robots (mainly search engine robots). It contains the information how you would like the robots to crawl and index your website on their own webpages. robots.txt contains the search bots or others bots name, directory list allowed or disallowed, indexing and crawling instructions for bots, time delay to crawl and index and even the sitemap URL. A full access or a full restriction, customized access or restriction, all can be imposed through robots.txt.

robots.txt is very important part for the SEO process. Your website directories will be crawled and indexed on search engine according to robots.txt instructions. So you need to add a robots.txt file in your website root directory. Write it properly, including your content enriched pages and other public pages, but exclude any pages which contain sensitive information. Remember robots.txt instruction to restrict access to your sensitive information of your page as it is not formidable on web page security grounds.
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Sitemap is a xml file which contains a full list of your websites URLs. It is used to include directories of your websites as the base for crawling and indexing of search engines and also for user access. The sitemap can help search engine bots to index your website faster and with deeper results. It is roughly the opposite of the robots.txt You can create a sitemap.xml with various free and paid services or you can write it by yourself (read the Learn more below, to see the inside about how to 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 URLs than you should create multiple sitemap files and use a sitemap index file as well.
2) Put your sitemap in the website root directory and add the URL of your sitemap into the robots.txt file.
3) Sitemap.xml can be compressed using grip for faster loading time.

Broken link : a broken link is an inaccessible link or a URL of a website. A higher rate of broken links has a negative impact on search engine ranking due to reduced link quality. It also has a bad impact on user experience if they run into a 404 page. There are several reasons for broken links, the most common 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 does not exists anymore. (Changing domain or site blocked or dysfunctional).
4) The content may have been hidden behind some firewalls 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.
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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.

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 are registered under single IP address, some search bots might label another website as a duplicate of one sites. This is IP canonicalization. Little bit like URL canonicalization. To solve this issue, you want to use redirects.
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