How to make DOM not to wait till javascript is downloaded and just open the page?

That's easy. Just add async attribute to the script tag. DOM will be ready before Javascript is downloaded, so the visitor will see contents of the page faster. It's always better, when your JavaScript is only used for user's interaction.

How to hide post footer in blogger all posts or archive view?

In Blogger's configuration you can choose what to show in the footer of your posts, but there are only global footer options for all views of your posts - view of one post, view of all posts list or archive. To hide blogger's post footer in all posts view and in archive view, use this JavaScript custom code. It is fast and super lightweight:

How to add custom HTML/JavaScript to Blogger blog?

No matter what you need - add your custom adsense or analytics code or make some new feature, if you need to add your custom HTML/JavaScript code it is easy to do without changing your blog's theme. Follow these steps:

What is the different between HTML CSS PHP and JavaScript?

That's a good question. It's very important to understand this different before starting to develop any website. So let's dive in:

HTML - it's like bones for your site. It says what to display on the user's browser: Tables, titles, paragraphs, lists, links etc. It is client-side language which is understood by browsers like Google Chrome, FireFox, Safari.