Add first pass of tags.md

Author: willcosgrove

handbook/tags.md

@@ -1 +1,120 @@
 # Tags
+
+All non-depreacted HTML tags are defined as instance methods on `Phlex::HTML`. They take keyword arguments which get turned into HTML attributes, and blocks which become content for the tag.
+
+## Attributes
+Attributes are normally symbol keys, which get dasherized. If you need a special attribute that has underscores in it, you can use a string key instead.
+
+::: code-group
+```ruby
+div(data_controller: "hello", "_special": "value") do
+  "Hello!"
+end
+```
+```html
+<div data-controller="hello" _special="value">
+  Hello!
+</div>
+```
+:::
+
+Alternatively, if you have multiple nested attributes (like several `data-*` attributes) you can use a hash:
+
+::: code-group
+```ruby
+div(data: { controller: "hello", action: "click->hello#show" }) do
+  "Hello!"
+end
+```
+```html
+<div data-controller="hello" data-action="click->hello#show">
+  Hello!
+</div>
+```
+:::
+
+The `class` and `style` attributes have special handling. If you use a hash value with the `class` key, it will work similarly to the Rails' `class_names` helper when passed a hash:
+
+::: code-group
+```ruby
+is_active = true
+is_disabled = false
+
+div(class: { active: is_active, disabled: is_disabled }) do
+  "Hello!"
+end
+```
+```html
+<div class="active">
+  Hello!
+</div>
+```
+:::
+
+If you use a hash value with the `style` key, it will be converted to a CSS string:
+
+::: code-group
+```ruby
+h1(style: { color: "red", font_size: "16px" }) do
+  "Hello!"
+end
+```
+```html
+<h1 style="color: red; font-size: 16px;">
+  Hello!
+</h1>
+```
+:::
+
+## Content
+
+Content is always passed as a block to the tag method. The block content works differently depending on whether or not other tag methods are called inside the block.
+
+If there are no other tag methods called inside the block, then the return value of the block is used as the content
+
+::: code-group
+```ruby
+div do
+  "Hello!"
+end
+```
+```html
+<div>
+  Hello!
+</div>
+```
+:::
+
+If there are other tag methods called inside the block, then the return value of the block is ignored. Instead, if you need to pass string content outside of a nested tag, you can use the `plain` method.
+
+::: code-group
+```ruby
+p do
+  strong { "Hello" }
+  plain " World!"
+end
+```
+```html
+<p>
+  <strong>Hello</strong> World!
+</p>
+```
+:::
+
+If we wrote that without the `plain` method, we would be missing the ` World!` part.
+
+::: code-group
+```ruby
+p do
+  strong { "Hello" }
+  " World!"
+end
+```
+```html
+<p>
+  <strong>Hello</strong>
+</p>
+```
+:::
+
+That is because the `p` tag's block has another tag inside of it, so it ignores the return value of the block.