Conform to w3c standard for HTML

@BarryGallagher đang làm issue này rồi.

Từ ngày 2/4/2019.

@elixier01 Here's the link to the validator

Passed the website to https://validator.w3.org/nu/?doc=https%3A%2F%2Fcode-co-op.github.io%2Flearning-web-development%2Findex.html

Produced the following errors
Warning: Consider adding a lang attribute to the html start tag to declare the language of this document.

From line 1, column 16; to line 2, column 6

For further guidance, consult Declaring the overall language of a page and Choosing language tags.

If the HTML checker has misidentified the language of this document, please file an issue report or send e-mail to report the problem.

Error: The main element must not appear as a descendant of the section element.

From line 45, column 9; to line 45, column 14
Error: The main element must not appear as a descendant of the main element.

From line 45, column 9; to line 45, column 14
Error: A document must not include more than one visible main element.

From line 45, column 9; to line 45, column 14
Error: The main element must not appear as a descendant of the section element.

From line 72, column 9; to line 72, column 14

Error: The main element must not appear as a descendant of the main element.

From line 72, column 9; to line 72, column 14

Error: A document must not include more than one visible main element.

From line 72, column 9; to line 72, column 14

Warning: Section lacks heading. Consider using h2-h6 elements to add identifying headings to all sections.

From line 71, column 7; to line 71, column 28

Error: The main element must not appear as a descendant of the section element.

From line 92, column 9; to line 92, column 14

Error: The main element must not appear as a descendant of the main element.

From line 92, column 9; to line 92, column 14

Error: A document must not include more than one visible main element.

From line 92, column 9; to line 92, column 14

Warning: Article lacks heading. Consider using h2-h6 elements to add identifying headings to all articles.

From line 97, column 7; to line 97, column 64