Before you start promoting your website in the Internet, you need to be sure that it looks good and works correctly.
Now a lot of different browsers and an incredible number of different gadgets, with different screen sizes.
And you should try to make sure that your users from any device can seen your site and it was convenient for them.
Correct HTML website layout
There are many standards that are used on the Internet and most likely it is impossible to meet all.
Unless you make a site that will use the oldest layout system. But this in the future will not lead to anything good.
But whatever standard you follow, it should be valid.
So it's better to agree with the rules of this standard.
In the network there is a website of the organization W3C, which develops and implements technological standards for the World Wide Web.
There are a lot of versions of HTML and CSS and much more. But what is the most pleasant for developers,
that they have the function of site verification (Online Validation Service).
Markup HTML Online Validation Service available here: validator.w3.org
There is compliance of your version of HTML with standards is checked. On the one hand, you can tell "Why I need this? Why they asked me to adapt to the some rules?"
Of course, the test is only advisory, and this is your website. You decide should you fix it according to the requirements of the standard or not.
Sometimes corrections entail a large amount of work and it is not so simple to bring them all in. But frequently the validator will show you mistakes or even typos, which are easy to fix.
In addition, the validator has 2 degrees of criticality: "Warning" and "Error".