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The new block won't actually appear on the homepage as an html block with matching menu and text colors until you add some content. Once the content is added and you refresh your homepage, the block will be there. You can add java and image maps too and you don't need to include and head, title, or body tags since it'll match the rest of the homepage blocks by default. Depending on what you have in mind, you might want to use the center tag first, just to make sure that everything lines up neatly within the html block.
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Finally, also keep in mind that if you create any size specific items ... resizing the columns within the builders section later on would also have an impact on those size specific items. So best be 100% certain about the page and column widths that you're using, before adding new content.
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Greetings from Germany