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Keep it simple. Forget all you know about tableless design and advanced CSS as HTML support in Email is awful. The worst offenders are the latest Outlook clients. So write your html as though you are still in the 90's.
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Use inline CSS. The web based email clients strip out all the information from the body tag up. So using inline CSS the best way to go. Remember though not all CSS is recognised in some Email clients due to security issues so try stick to HTML for formatting that includes using tables for layout design.
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Images do not always display by default in the majority of clients and most of the time the alt tags are not displayed. So make sure all your content is text based. Another area to watch out for is the layout of the email. Without inserting the width and height settings the blank placeholders for your images will throw out your layout.
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HTML Email sizing. Best practice would dictate that emails use percentages for sizing as you have no idea on the dimensions of the email client. However if the design is fixed layout then try between 500px and 600px.
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Avoid JavaScript. The vast majority of clients will remove it.
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Spacing is very tricky as css margin and padding does not always work stick to td with width and height settings.
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Links should use the target=_blank. Online email clients do not want to lose their session when clicking a new link.
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Background images. Don't bother trying to setup background images as most clients will not display them. Rather slice your images and around the content.