Add styles

xx.md

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+
+NIP-XX
+======
+
+Out-of-line Styles
+------------------
+
+`draft` `optional`
+
+Out-of-line styles are text formatting data not inlined in the
+event's `.content` field but placed into `st` (style) tags.
+
+## Motivation
+
+With markup languages such as Markdown, AsciiDoc and HTML, clients
+would be required to add an ever growing number of parsers to
+support textual event kinds.
+
+If keeping `.content` field limited to plaintext,
+clients can show text with no extraneous markup delimiter chars
+that could impact readability.
+
+When a client chooses to support formatting, it can
+add parser code just once and it will work for all textual
+events. If the parser in incomplete, there is not much
+problem because the raw text inside `.content` is perfectly readable.
+
+## Writing Text
+
+A client should offer text formatting in any markup language,
+for example, WYSIWYG powered by HTML, or Markdown editor.
+However, before saving the event it should transform the resulting
+text into an unformatted `.content` with added style tags.
+
+## Style Tags
+
+A style tag delimits an inclusive character range
+and specifies a style to be applied using the following syntax:
+`["st", "<starting-character-index>:<ending-character-index>", "<style-name>"]`
+
+### Style Names
+
+- `h<1-to-9>`: Header (h1) or subheader
+- `strong`: Strong importance; should use heavier weight font.
+- `em`: Emphasis; should use italic type.
+- `small`: Small print, like copyright and legal text. Use smaller font-size.
+- `strike`: Stricken text. Should remove leading and trailing `-`.
+- `del`: Deleted text, such as after a git commit. Should remove leading `-` and spaces.
+- `ins`: Inserted text, such as after a git commit. Should remove leading `+` and spaces.
+- `sub`: Text should be displayed as subscript.
+- `sup`: Text should be displayed as superscript.
+- `mark` Text should be highlighted with a different background color.
+- `quote`: Quoted text. Should remove leading and trailing `"`.
+- `dsv` or `dsv:<delimiter-char-sequence>`: Delimiter separated value. The default delimiter is `|`.
+Another delimiter that doesn't clash with the actual values may be choosed, e.g.: `dsv:#-#`. Values should be trimmed.
+- `code` or `code:<lowercase-language-name-without-space>)`: Displays code, optionally in some language.
+
+Styles may be combined, for instance, a `dsv`'s header may have its char range set to `h1`.
+
+## Example
+
+```js
+{
+  "tags": [
+    ["st", "0:7", "h1"],
+    ["st", "10:29", "h2"],
+    ["st", "51:51", "h1"],
+    ["st", "10:71", "dsv"]
+  ],
+  "content": "My Title
+
+  |    B    |    C
+D | Item B1 | Item C1
+E | Item B2 | Item C2
+
+  • Bullet points don't need style tags
+  • I simply used spaces to ident these
+  1) Here too
+  1.1) Example"
+  // other fields
+}
+```