By "don't close your tags", I mean "don't close your tags when the HTML spec does not require you to." You still need to close your <a>, <div>, and <span> tags. You don't need to close your <li>, <th>, <tr>, <td>, <dl>, <dt>, and several other tags. You don't even need to open your <html> and <head> tags, if you're not doing funky stuff like putting comments in them.
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By "don't close your tags", I mean "don't close your tags when the HTML spec does not require you to." You still need to close your <a>, <div>, and <span> tags. You don't need to close your <li>, <th>, <tr>, <td>, <dl>, <dt>, and several other tags. You don't even need to open your <html> and <head> tags, if you're not doing funky stuff like putting comments in them.