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, which signifies any descendant of the previous selector. If you want to select a span with both classes text and big that is a descendant of an h2 tag, which is a descendant of a p tag with class title, which is a descendant of a div with class filters, you should write it like this:

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

This will select the correct elements and apply a font size of 20px.