Is following CSS syntax valid?body { color: #FF0000;}div.filters p.title h2 span.text.big { font-size: 20px;}

The CSS syntax you provided is partially correct. The first part is valid:

body {    
    color: #FF0000;
}

This will set the text color of the entire body to red.

However, the second part is not valid:

div.filters p.title h2 span.text.big {    
    font-size: 20px;
}

In CSS, you cannot nest selectors in this way. Each selector should be separated by a space or a greater than sign (>). The correct syntax would look something like this:

div.filters p.title > h2 > span.text.big {    
    font-size: 20px;
}

This will select a span with the classes text and big that is a direct child of an h2 element, which is a direct child of a p element with the class title, which is a direct child of a div with the class filters.

If you want to select an element that is a descendant (not necessarily a direct child), you can use a space:

div.filters p.title h2 span.text.big {    
    font-size: 20px;
}

This will select a span with the classes text and big that is a descendant of an h2 element, which is a descendant of a p element with the class title, which is a descendant of a div with the class filters.