Is following CSS syntax valid?body { color: #FF0000;}h3,div.full_textdiv.small_text h4div.filters p.title { font-size: 20px;}

No, the CSS syntax you provided is not valid. The problem is with the selectors. In CSS, when you want to select multiple elements, you need to separate each selector with a comma. Here is the corrected version:

body {    
    color: #FF0000;
}

h3,
div.full_text,
div.small_text h4,
div.filters p.title {    
    font-size: 20px;
}

In this corrected version, the body will have a color of #FF0000, and the h3, div with a class of full_text, h4 inside a div with a class of small_text, and p with a class of title inside a div with a class of filters will all have a font size of 20px.