Widget tag name with colon

[incur]

Steps to reproduce

1. Add widget with tag name 'cnn:news' widget template is '<cnn:news>Some news</cnn:news>'

2. In source mode ckeditor add next html:

<h1>111</h1> <cnn:news>Some news</cnn:news>

<h1>111</h1> <cnn:news>Some news</cnn:news>

<h1>111</h1> <cnn:news>Some news</cnn:news>

3. Push up button "source" and push down again
4. And we see:

<h1>111</h1> <cnn:news>Some news</cnn:news>

<h1>111</h1>

<h1>111</h1> <cnn:news>Some news</cnn:news>

5. There were only two widget tags

[Jakub Ś]

CKEditor doesn't support custom elements by default. In order to make them work, dtd needs to be modified (source code for dtd), ACF needs to be extended and new editor needs to be built. Please see: ​https://stackoverflow.com/questions/16066556/ckeditor-how-to-allow-for-inserthtmlcustomtag-myattr-value-customtag/16068708#16068708

Direct Source Code modification is required for block-level tags. For inline tags dtd can be modified before first editor initialization. Please see #14418 for code example.

This issue is about making DTD work with namespaces like cnn:.

[Jakub Ś]

Related issues: #11562, #10340