HTML - it's like bones for your site. It says what to display on the user's browser: Tables, titles, paragraphs, lists, links etc. It is client-side language which is understood by browsers like Google Chrome, FireFox, Safari.
CSS - these are design rules. It says to the browser how to display HTML elements. Defines font size, color, table's border, link's color etc. It is client-side language which is understood by browsers like Google Chrome, FireFox, Safari
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