Important things to remember for UI developers & Software developers while writing HTML.
1. Always Close Your Tags: Many chose to leave off the closing <li> tags as well. By today’s standards, this is simply bad practice and should be 100% avoided. Always, always close your tags. Otherwise, you’ll encounter validation and glitch issues at every turn.
2. Declare the Correct DocType: The DOCTYPE goes before the opening html tag at the top of the page and tells the browser whether the page contains HTML, XHTML, or a mix of both, so that it can correctly interpret the markup.
3. Never Use Inline Styles (Except Newsletters) : It looks harmless enough. However, this points to an error in your coding practices.
4. Place all External CSS Files Within the Head Tag: While researching performance at Yahoo!, It was discovered that moving style sheets to the document HEAD makes pages appear to be loading faster. This is because putting style sheets in the HEAD allows the page to render progressively.
7. Validate Continuously
9. Use Firebug! Many users only take advantage of about 20% of Firebug’s capabilities. You’re truly doing yourself a disservice. Take a couple hours and scour the web for every worthy tutorial you can find on the subject.
10. Keep Your Tag Names Lowercase
11. Use H1 – H6 Tags : It’s best practice to use all six of these tags. For semantic and SEO reasons, force yourself to replace that P tag with an H6 when appropriate.
12. If Building a Blog, Save the H1 for the Article Title
Source : https://experiencedesignideas.wordpress.com/2012/11/22/html-best-practices-part-1-3-part-series/