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.
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".
And if with the most of "Warnings" website still work without problem, then I recommend to consider the "Errors" in more detail. You should understand that browsers also use these standards, which means if you have "Errors", that in some browsers your website may not look exactly as you expect.
Let me remind you that compliance with standards is only a recommendation. But HTML and CSS are languages. And when someone speaks wrong in this language, this attracts attention at least (in the bad sense of the word) and in the worst case this increases your chances of being incomprehensible by search engine. It's very close to me, because English is not my native language. And I always learn it. Therefore, if you notice some mistakes-typos, so I will be glad if you inform and correct me.
So, next case is CSS. What is it? CSS is meaning "Cascading style sheets". This is more about style of websites. CSS also has many versions. And of course, then newer version, the fewer browsers it supports. At the same time, the newer version of CSS you used on the website give you wider possibilities and the easier creating some design things.
The CSS is checked on this site: jigsaw.w3.org/css-validator/
There will also be "Errors" and "Warnings". But unfortunately for making the perfect CSS you need to work very hard. The fact is, in CSS has some elements that are written differently for different browsers. And of course those that are suitable for a single browser are not suitable for another.
For example, the element "border-radius". In order to write it for all browsers, it will look like this:
-moz-border-radius: 10px; /* for Firefox */
-ms-border-radius: 10px; /* for Internet Explorer */
-o-border-radius: 10px; /* for Opera */
-webkit-border-radius: 10px; /* for Chrome, Safari */
This allows properties to be set specific to each individual browser in order for inconsistencies between implementations to be safely accounted for.
If you get a large list of "Errors" and "Warnings" from this check. You don't have to worry about most of these "problems". Carefully look at the list. Maybe you just typo somewhere. Remember that CSS is just the display styles of the page, i.e. for search engines it does not really matter. It's more about feeling visitors when they see your website.
In the future, I'll tell you about the structural layout of the site, but this is a separate topic.
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