chore: add docs site

docs/guide/special.md

@@ -0,0 +1,108 @@
+# Special cases
+
+::: tip Read first
+Understanding [How is metaInfo resolved?](/faq/#concatenate-metadata) is probably a prerequisite for these cases
+:::
+
+## Remove parent property by child
+
+If a child returns `null` as content value then the parent metaInfo property with the same `vmid` will be ignored
+
+:::tip Content value
+With content value we mean the following value of a `metaInfo` property:
+- the value of a key for `object` types as [`htmlAttrs`](/api/#htmlattrs)
+- the value of `[contentKeyName]` or `innerHTML` keys for `collection` types as [`meta`](/api/#meta)
+:::
+
+The following might be a bit-farfetched, but its just an example
+```js
+// parent
+metaInfo: {
+  style: [{
+    vmid: 'page-load-overlay',
+    innerHTML: `
+      body div.loading {
+        z-index: 999;
+        background-color: #0f0f0f;
+        opacity: 0.9;
+      }
+    `,
+  }]
+}
+
+// dynamically loaded child
+metaInfo() {
+  const style = this.cssTexts
+  return { style }
+},
+data() {
+  return {
+    this.cssTexts: []
+  }
+},
+mounted() {
+  this.cssTexts.push({
+    vmid: 'page-load-overlay',
+    innerHTML: null
+  })
+}
+```
+
+## Use child property conditionally
+
+If you wish to use a child property conditionally and use the parents' property as default value, make sure the child returns `undefined` as content value
+
+:::tip Content value
+With content value we mean the following value of a `metaInfo` property:
+- the value of a key for `object` types as [`htmlAttrs`](/api/#htmlattrs)
+- the value of `[contentKeyName]` or `innerHTML` keys for `collection` types as [`meta`](/api/#meta)
+:::
+
+The below example will still show a description when the GET in the child fails
+```js
+// parent
+metaInfo: {
+  meta: [{
+    vmid: 'description',
+    name: 'description',
+    content: 'my standard description',
+  }]
+}
+
+// child
+metaInfo() {
+  return {
+    meta: [{
+      vmid: 'description',
+      name: 'description',
+      content: this.description,
+    }]
+  }
+},
+data() {
+  return {
+    description: undefined
+  }
+},
+methods: {
+  getDescription() {
+    this.description = this.$axios.get()
+    if (!this.description) {
+      // if GET request failed or returned empty,
+      // explicitly set to undefined so the parents'
+      // default description is used
+      this.description = undefined
+    }
+  }
+}
+```
+
+## Boolean attributes
+
+`vue-meta` maintains a [list](https://github.com/nuxt/vue-meta/blob/master/src/shared/constants.js) of attributes which are boolean attributes according to the HTML specs (and some extra). Whatever value you will pass to these attributes, they will be rendered as a boolean attribute.<sup>*</sup>
+
+<sup>*</sup><small>Except for the special values `undefined` and `null`, see above</small>
+
+:::tip Note
+Prior to `v2.0` any attribute key with `undefined` as value was rendered as boolean attribute. This has been removed as packagers often remove object properties with an `undefined` value as given `a = {}` then `a.a === undefined`
+:::