diff --git a/runtime/doc/develop.txt b/runtime/doc/develop.txt
index ed4e9e3..d90e4a3 100644
--- a/runtime/doc/develop.txt
+++ b/runtime/doc/develop.txt
@@ -1,4 +1,4 @@
-*develop.txt*   For Vim version 8.0.  Last change: 2017 Jul 31
+*develop.txt*   For Vim version 8.0.  Last change: 2018 Apr 17
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -185,9 +185,49 @@
 C COMPILER						*style-compiler*
 
 The minimal C compiler version supported is C89, also known as ANSI C.
-Later standards don't add much and C89 is the widest supported.
+Later standards, such as C99, are not widely supported, or at least not 100%
+supported.  Therefore we use only some of the C99 features and disallow some
+(at least for now).
 
-One restriction that this implies: no // comments, only /* comments */.
+Please don't make changes everywhere to use the C99 features, it causes merge
+problems for existing patches.  Only use them for new and changed code.
+
+Comments ~
+
+Traditionally Vim uses /* comments */.  We intend to keep it that way,
+especially for file and function headers.  For new code or lines of code that
+change, it is allowed to use // comments.  Especially when it comes after
+code:
+	int some_var;  // single line comment useful here
+
+Enums ~
+
+The last item in an enum may have a trailing comma.  C89 didn't allow this.
+
+Types ~
+
+"long long" is allowed and can be expected to be 64 bits.  Use %lld in printf
+formats.  Also "long long unsigned" with %llu.
+
+Flexible array members ~
+
+This is an array without size, used as the last member of a struct.  Vim used
+to have an array of size one, which causes trouble with FORTIFY_SOURCE. Using
+an "unsized array" is the intended use, we will change all of them.
+	struct some_stuff {
+	   size_t  length;
+	   char    payload[];  // will have size "length"
+	};
+
+Not to be used ~
+
+These C99 features are not to be used, because not enough compilers support
+them:
+- Declaration after Statements (MSVC 2012 does not support it).  All
+  declarations need to be at the start of the block.
+- Variable length arrays (even in C11 this is an optional feature).
+- _Bool and _Complex types.
+- "inline" (it's hardly ever needed, let the optimizer do its work)
 
 
 USE OF COMMON FUNCTIONS					*style-functions*