The "alt" attribute provides a text equivalent for the image. If the browser cannot display an image the alt description will be given in its place. Furthermore, some visitors cannot see images as they might be blind in which the alt tag provides a valuable image description. Finally, search engines utilize the alt attribute for image search indexing.
The <TITLE> element provides a short piece of text describing the document. The title is very important as it shows in the window title bars, bookmarks and search results. Title should be between 60 to 80 characters long.
The keywords attribute was utilized by search engines to provide more accurate search results. Due to the over usage and spamming the keywords attribute has been phased out and is no longer relevant. Google doesn't use the "keywords" meta tag in web search ranking.
Original content is by far the single most important element to search engines. Low Text to HTML ratio indicates little content for search engines to index. We consider it to be good practice to have a Text to HTML ratio of at least 10% and ideal above 20%
It is not recommended to use frames or iframes because they can cause problems for search engines. It is best to avoid frames and inline frames whenever possible (Read Article). If frames must be utilized consider <noframes> tag.
The <h1> to <h6> tags are used to define HTML headings. The <h1> tag should hold the title describing the content of a specific page therefore only one h1 tag is needed per page. There can be any number of h2 - h6 tags but they should be added in order of importance.
The declaration tells the web browser about what version of HTML the page is written in. It is good practice to always add the declaration to the HTML documents, so that the browser knows what type of document to expect.
Sitemaps inform search engines about pages available for crawling. In its simplest form, a Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL. XML sitemap should help search engines to crawl the site more intelligently.
The Favicon is a small icon associated with a website. The Favicon is important because it is displayed next to the website's URL in the address bar of the browser as well as in bookmarks and shortcuts.
Protect Webserver directories from unwanted browsing. To enhance security, directory browsing should be disabled unless you have a specific reason to enable it. If you enable directory browsing, make sure that you only enable it on the particular directory or directories that you want to share.