diff --git a/src/components/core.ts b/src/components/core.ts
index f4071bf6..95b3b52b 100644
--- a/src/components/core.ts
+++ b/src/components/core.ts
@@ -251,7 +251,7 @@ export default class Core {
     }
 
     if (holder && _.isObject(holder) && !$.isElement(holder)) {
-      throw Error('holder as HTMLElement if provided must be inherit from Element class.');
+      throw Error('«holder» value must be an Element node');
     }
   }
 
diff --git a/test/testcases.md b/test/testcases.md
new file mode 100644
index 00000000..00080a09
--- /dev/null
+++ b/test/testcases.md
@@ -0,0 +1,82 @@
+# Editor.js specs
+
+This document will describe various test cases of the editor.js functionality. Features will be organized by modules. Cases covered by tests should be marked by the checkmark.
+
+## Configuration
+
+- [ ] Zero configuration
+  - [ ] Editor.js should be initialized on the element with the default `editorjs` id.
+  - [ ] Editor.js should throw an error in case when there is no element with `editorjs` id.
+  - [ ] Editor.js should be initialized with the Paragraph tool only.
+  - [ ] The Inline Toolbar of the Paragraph tool should contain all default Inline Tools - `bold`, `italic`, `link`.
+
+- [ ] `holder` property
+  - [ ] Editor.js should be initialized on the element with passed via `holder` property.
+  - [ ] Editor.js should throw an error if passed `holder` value is not an Element node.
+
+- [ ] `autofocus` property
+  - [ ] With the empty editor
+    - [ ] If `true` passed, the caret should be placed to the first empty block.
+    - [ ] If `false` passed, the caret shouldn't be placed anywhere.
+    - [ ] If omitted, the caret shouldn't be placed anywhere.
+  - [ ] With the not-empty editor
+    - [ ] If `true` passed, the caret should be placed to the end of the last block.
+    - [ ] If `false` passed, the caret shouldn't be placed anywhere.
+    - [ ] If omitted, the caret shouldn't be placed anywhere.
+
+- [ ] `placeholder` property
+  - [ ] With the empty editor
+    - [ ] If `string` passed, the string should be placed as a placeholder to the first empty block only.
+    - [ ] If `false` passed, the first empty block should be placed without a placeholder.
+    - [ ] If omitted, the first empty block should be placed without a placeholder.
+
+- [ ] `minHeight` property
+  - [ ] If `number` passed, the height of the editor's bottom area from the last Block should be the `number`.
+  - [ ] If omitted the height of editor's bottom area from the last Block should be the default `300`.
+
+- [ ] `logLevel` property
+  - [ ] If `VERBOSE` passed, the editor should output all messages to the console.
+  - [ ] If `INFO` passed, the editor should output info and debug messages to the console.
+  - [ ] If `WARN` passed, the editor should output only warning messages to the console.
+  - [ ] If `ERROR` passed, the editor should output only error messages to the console.
+  - [ ] If omitted, the editor should output all messages to the console.
+
+- [ ] `defaultBlock` property
+  - [ ] If `string` passed
+    - [ ] If passed `string` in the `tools` option, the passed tool should be used as the default tool.
+    - [ ] If passed `string` not in the `tools` option, the Paragraph tool should be used as the default tool.
+  - [ ] If omitted the Paragraph tool should be used as default tool.
+
+- [ ] `sanitizer` property
+  - [ ] If `object` passed
+    - [ ] The Editor.js should clean the HTML tags according to mentioned configuration.
+  - [ ] If omitted the Editor.js should be initialized with the default `sanitizer` configuration, which allows the tags like `paragraph`, `anchor`, and `bold` for cleaning HTML.
+
+- [ ] `tools` property
+  - [ ] If omitted
+    - [ ] Editor.js should be initialized with the Paragraph tool only.
+  - [ ] If `object` passed
+    - [ ] Editor.js should be initialized with all the passed tools.
+    - [ ] The keys of the object should be represented as `type` fields for corresponded blocks in output JSON
+    - [ ] If value is a JavaScript class
+      - [ ] This class should be used as a tool
+    - [ ] If value is an `object`
+      - [ ] Checking the `class` property
+        - [ ] If omitted, the tool should be skipped with a warning in a console.
+        - [ ] If existed, the value of the `class` property should be used as a tool
+      - [ ] Checking the `config` property
+        - [ ] If `object` passed Editor.js should initialize `tool` and pass this object as `config` parameter of the tool's constructor
+      - [ ] Checking the `shortcut` property
+        - [ ] If `string` passed Editor.js should append the `tool` when such keys combination executed.
+      - [ ] Checking the `inilineToolbar` property
+        - [ ] If `true`
+          - [ ] Editor.js should show the Inline Toolbar for this tool with [common](https://editorjs.io/configuration#inline-toolbar-order) settings.
+        - [ ] If `false`
+          - [ ] Editor.js should not show the Inline Toolbar for this tool.
+        - [ ] If `array`
+          - [ ] Editor.js should show the Inline Toolbar for this tool with a passed list of tools and their order.
+        - [ ] If omitted
+          - [ ] Editor.js should not show the Inline Toolbar for this tool.
+      - [ ] Checking the `toolbox` property
+        - [ ] If it contains `title`, this title should be used as a tool title
+        - [ ] If it contains `icon`, this HTML code (maybe SVG) should be used as a tool icon
diff --git a/types/configs/editor-config.d.ts b/types/configs/editor-config.d.ts
index c892d7cb..a9353f57 100644
--- a/types/configs/editor-config.d.ts
+++ b/types/configs/editor-config.d.ts
@@ -11,7 +11,7 @@ export interface EditorConfig {
   holderId?: string | HTMLElement;
 
   /**
-   * Element where Editor will be append
+   * Element where Editor will be appended
    */
   holder?: string | HTMLElement;