CHROMIUM_map_sub died along with NaCl.

src/Magnum/Buffer.h

@@ -395,8 +395,7 @@ class MAGNUM_EXPORT Buffer: public AbstractObject {
          *
          * @see @ref map(MapAccess)
          * @m_enum_values_as_keywords
-         * @requires_es_extension Extension @extension{OES,mapbuffer} or
+         * @requires_es_extension Extension @extension{OES,mapbuffer}
-         *      @extension{CHROMIUM,map_sub}
          * @requires_gles Buffer mapping is not available in WebGL.
          * @deprecated_gl Prefer to use @ref map(GLintptr, GLsizeiptr, MapFlags)
          *      instead, as it has more complete set of features.

src/Magnum/Text/Renderer.h

@@ -193,11 +193,10 @@ that doesn't recreate everything on each text change:
 @section Text-Renderer-required-opengl-functionality Required OpenGL functionality
 
 Mutable text rendering requires @extension{ARB,map_buffer_range} on desktop
-OpenGL (also part of OpenGL ES 3.0). If neither @extension{EXT,map_buffer_range}
+OpenGL (also part of OpenGL ES 3.0). If @extension{EXT,map_buffer_range} is not
-nor @extension{CHROMIUM,map_sub} is not available in ES 2.0, at least
+available in ES 2.0, at least @extension{OES,mapbuffer} must be supported for
-@extension{OES,mapbuffer} must be supported for asynchronous buffer updates.
+asynchronous buffer updates. There is no similar extension in WebGL, thus plain
-There is no similar extension in WebGL, thus plain (and slow) buffer updates
+(and slow) buffer updates are used there.
-are used there.
 
 @see @ref Renderer2D, @ref Renderer3D, @ref AbstractFont,
     @ref Shaders::AbstractVector