diff --git a/src/md4c.c b/src/md4c.c
index 43d7655..00f1263 100644
--- a/src/md4c.c
+++ b/src/md4c.c
@@ -245,7 +245,8 @@ enum MD_LINETYPE_tag {
     MD_LINE_HTML,
     MD_LINE_TEXT,
     MD_LINE_TABLE,
-    MD_LINE_TABLEUNDERLINE
+    MD_LINE_TABLEUNDERLINE,
+    MD_LINE_BOX_INFO
 };
 typedef enum MD_LINETYPE_tag MD_LINETYPE;
 
@@ -4869,11 +4870,55 @@ abort:
     return ret;
 }
 
+static int
+md_setup_box_detail(MD_CTX* ctx, const MD_BLOCK* block, MD_BLOCK_BOX_DETAIL* det,
+                    MD_ATTRIBUTE_BUILD* type_build, MD_ATTRIBUTE_BUILD* title_build)
+{
+    const MD_LINE* line = (const MD_LINE*)(block + 1);
+
+    OFF beg = line->beg;
+    OFF end = line->end;
+    OFF sep;
+    int ret = 0;
+
+    /* Trim initial spaces. */
+    while(beg < ctx->size  &&  ISBLANK(beg))
+        beg++;
+
+    /* Trim trailing spaces. */
+    while(end > beg  &&  ISBLANK(end-1))
+        end--;
+
+    /* Look for type. */
+    sep = beg;
+    while(sep < end  &&  !ISBLANK(sep)  &&  CH(sep) != ':')
+        sep++;
+    if (sep < end  &&  CH(sep) == ':') {
+        /* Build type string attribute. */
+        MD_CHECK(md_build_attribute(ctx, STR(beg), sep - beg, 0, &det->type, type_build));
+        beg = sep + 1;
+        while(beg < end  &&  ISBLANK(beg))
+            beg++;
+    } else {
+        /* Build empty type string attribute. */
+        MD_CHECK(md_build_attribute(ctx, NULL, 0, 0, &det->type, type_build));
+    }
+
+    /* Build info string attribute. */
+    MD_CHECK(md_build_attribute(ctx, STR(beg), end - beg, 0, &det->title, title_build));
+
+abort:
+    return ret;
+}
+
 static int
 md_process_all_blocks(MD_CTX* ctx)
 {
     int byte_off = 0;
     int ret = 0;
+    MD_ATTRIBUTE_BUILD type_build;
+    MD_ATTRIBUTE_BUILD title_build;
+    int clean_box_builds = FALSE;
 
     /* ctx->containers now is not needed for detection of lists and list items
      * so we reuse it for tracking what lists are loose or tight. We rely
@@ -4887,6 +4932,7 @@ md_process_all_blocks(MD_CTX* ctx)
             MD_BLOCK_UL_DETAIL ul;
             MD_BLOCK_OL_DETAIL ol;
             MD_BLOCK_LI_DETAIL li;
+            MD_BLOCK_BOX_DETAIL box;
         } det;
 
         switch(block->type) {
@@ -4907,6 +4953,16 @@ md_process_all_blocks(MD_CTX* ctx)
                 det.li.task_mark_offset = (OFF) block->n_lines;
                 break;
 
+            case MD_BLOCK_BOX:
+                memset(&det.box, 0, sizeof(MD_BLOCK_BOX_DETAIL));
+                if(block->flags & MD_BLOCK_CONTAINER_OPENER) {
+                    memset(&type_build, 0, sizeof(MD_ATTRIBUTE_BUILD));
+                    memset(&title_build, 0, sizeof(MD_ATTRIBUTE_BUILD));
+                    clean_box_builds = TRUE;
+                    MD_CHECK(md_setup_box_detail(ctx, block, &det.box, &type_build, &title_build));
+                }
+                break;
+
             default:
                 /* noop */
                 break;
@@ -4916,7 +4972,7 @@ md_process_all_blocks(MD_CTX* ctx)
             if(block->flags & MD_BLOCK_CONTAINER_CLOSER) {
                 MD_LEAVE_BLOCK(block->type, &det);
 
-                if(block->type == MD_BLOCK_UL || block->type == MD_BLOCK_OL || block->type == MD_BLOCK_QUOTE)
+                if(block->type == MD_BLOCK_UL || block->type == MD_BLOCK_OL || block->type == MD_BLOCK_QUOTE || block->type == MD_BLOCK_BOX)
                     ctx->n_containers--;
             }
 
@@ -4926,12 +4982,16 @@ md_process_all_blocks(MD_CTX* ctx)
                 if(block->type == MD_BLOCK_UL || block->type == MD_BLOCK_OL) {
                     ctx->containers[ctx->n_containers].is_loose = (block->flags & MD_BLOCK_LOOSE_LIST);
                     ctx->n_containers++;
-                } else if(block->type == MD_BLOCK_QUOTE) {
+                } else if(block->type == MD_BLOCK_QUOTE || block->type == MD_BLOCK_BOX) {
                     /* This causes that any text in a block quote, even if
                      * nested inside a tight list item, is wrapped with
                      * <p>...</p>. */
                     ctx->containers[ctx->n_containers].is_loose = TRUE;
                     ctx->n_containers++;
+
+                    /* box block actually contains an info string line */
+                    if(block->type == MD_BLOCK_BOX)
+                        byte_off += block->n_lines * sizeof(MD_LINE);
                 }
             }
         } else {
@@ -4949,6 +5009,10 @@ md_process_all_blocks(MD_CTX* ctx)
     ctx->n_block_bytes = 0;
 
 abort:
+    if(clean_box_builds) {
+        md_free_attribute(ctx, &type_build);
+        md_free_attribute(ctx, &title_build);
+    }
     return ret;
 }
 
@@ -5185,6 +5249,35 @@ abort:
     return ret;
 }
 
+static int
+md_push_box_bytes(MD_CTX* ctx, MD_LINE_ANALYSIS* analysis, unsigned flags)
+{
+    MD_BLOCK* block;
+    MD_LINE* line;
+    int ret = 0;
+
+    MD_CHECK(md_end_current_block(ctx));
+
+    block = (MD_BLOCK*) md_push_block_bytes(ctx, sizeof(MD_BLOCK));
+    if(block == NULL)
+        return -1;
+
+    block->type = MD_BLOCK_BOX;
+    block->flags = flags;
+    block->data = 0;
+    block->n_lines = 0;
+
+    line = (MD_LINE*) md_push_block_bytes(ctx, sizeof(MD_LINE));
+    if(line == NULL)
+        return -1;
+    line->beg = analysis->beg;
+    line->end = analysis->end;
+    block->n_lines++;
+
+abort:
+    return ret;
+}
+
 
 
 /***********************
@@ -5584,6 +5677,9 @@ md_is_container_compatible(const MD_CONTAINER* pivot, const MD_CONTAINER* contai
     if(container->ch == _T('>'))
         return FALSE;
 
+    if(container->ch == _T('!'))
+        return FALSE;
+
     if(container->ch != pivot->ch)
         return FALSE;
     if(container->mark_indent > pivot->contents_indent)
@@ -5615,7 +5711,7 @@ md_push_container(MD_CTX* ctx, const MD_CONTAINER* container)
 }
 
 static int
-md_enter_child_containers(MD_CTX* ctx, int n_children)
+md_enter_child_containers(MD_CTX* ctx, int n_children, MD_LINE_ANALYSIS* line)
 {
     int i;
     int ret = 0;
@@ -5651,6 +5747,10 @@ md_enter_child_containers(MD_CTX* ctx, int n_children)
                 MD_CHECK(md_push_container_bytes(ctx, MD_BLOCK_QUOTE, 0, 0, MD_BLOCK_CONTAINER_OPENER));
                 break;
 
+            case _T('!'):
+                MD_CHECK(md_push_box_bytes(ctx, line, MD_BLOCK_CONTAINER_OPENER));
+                break;
+
             default:
                 MD_UNREACHABLE();
                 break;
@@ -5692,6 +5792,11 @@ md_leave_child_containers(MD_CTX* ctx, int n_keep)
                                 0, MD_BLOCK_CONTAINER_CLOSER));
                 break;
 
+            case _T('!'):
+                MD_CHECK(md_push_container_bytes(ctx, MD_BLOCK_BOX, 0,
+                                0, MD_BLOCK_CONTAINER_CLOSER));
+                break;
+
             default:
                 MD_UNREACHABLE();
                 break;
@@ -5725,6 +5830,18 @@ md_is_container_mark(MD_CTX* ctx, unsigned indent, OFF beg, OFF* p_end, MD_CONTA
         return TRUE;
     }
 
+    /* Check for block box mark. */
+    if((ctx->parser.flags & MD_FLAG_BOX) && CH(off) == _T('!')) {
+        off++;
+        p_container->ch = _T('!');
+        p_container->is_loose = FALSE;
+        p_container->is_task = FALSE;
+        p_container->mark_indent = indent;
+        p_container->contents_indent = indent + 1;
+        *p_end = off;
+        return TRUE;
+    }
+
     /* Check for list item bullet mark. */
     if(ISANYOF(off, _T("-+*"))  &&  (off+1 >= ctx->size || ISBLANK(off+1) || ISNEWLINE(off+1))) {
         p_container->ch = CH(off);
@@ -5807,8 +5924,11 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
     while(n_parents < ctx->n_containers) {
         MD_CONTAINER* c = &ctx->containers[n_parents];
 
-        if(c->ch == _T('>')  &&  line->indent < ctx->code_indent_offset  &&
-            off < ctx->size  &&  CH(off) == _T('>'))
+        if((c->ch == _T('>')  &&  line->indent < ctx->code_indent_offset  &&
+             off < ctx->size  &&  CH(off) == _T('>'))
+                ||
+           (c->ch == _T('!')  &&  line->indent < ctx->code_indent_offset  &&
+             off < ctx->size  &&  CH(off) == _T('!')))
         {
             /* Block quote mark. */
             off++;
@@ -5822,7 +5942,8 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
 
             line->beg = off;
 
-        } else if(c->ch != _T('>')  &&  line->indent >= c->contents_indent) {
+        } else if(c->ch != _T('>')  &&  c->ch != _T('!')
+                &&  line->indent >= c->contents_indent) {
             /* List. */
             line->indent -= c->contents_indent;
         } else {
@@ -5836,7 +5957,9 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
         /* Blank line does not need any real indentation to be nested inside
          * a list. */
         if(n_brothers + n_children == 0) {
-            while(n_parents < ctx->n_containers  &&  ctx->containers[n_parents].ch != _T('>'))
+            while(n_parents < ctx->n_containers
+                    &&  ctx->containers[n_parents].ch != _T('>')
+                    &&  ctx->containers[n_parents].ch != _T('!'))
                 n_parents++;
         }
     }
@@ -5911,7 +6034,8 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
                 line->type = MD_LINE_BLANK;
                 ctx->last_line_has_list_loosening_effect = (n_parents > 0  &&
                         n_brothers + n_children == 0  &&
-                        ctx->containers[n_parents-1].ch != _T('>'));
+                        ctx->containers[n_parents-1].ch != _T('>') &&
+                        ctx->containers[n_parents-1].ch != _T('!'));
 
     #if 1
                 /* See https://github.com/mity/md4c/issues/6
@@ -5926,6 +6050,7 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
                  * item can begin with at most one blank line."
                  */
                 if(n_parents > 0  &&  ctx->containers[n_parents-1].ch != _T('>')  &&
+                   ctx->containers[n_parents-1].ch != _T('!') &&
                    n_brothers + n_children == 0  &&  ctx->current_block == NULL  &&
                    ctx->n_block_bytes > (int) sizeof(MD_BLOCK))
                 {
@@ -5945,6 +6070,7 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
             ctx->last_line_has_list_loosening_effect = FALSE;
             if(ctx->last_list_item_starts_with_two_blank_lines) {
                 if(n_parents > 0  &&  ctx->containers[n_parents-1].ch != _T('>')  &&
+                   ctx->containers[n_parents-1].ch != _T('!') &&
                    n_brothers + n_children == 0  &&  ctx->current_block == NULL  &&
                    ctx->n_block_bytes > (int) sizeof(MD_BLOCK))
                 {
@@ -6036,7 +6162,8 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
            md_is_container_mark(ctx, line->indent, off, &off, &container))
         {
             if(pivot_line->type == MD_LINE_TEXT  &&  n_parents == ctx->n_containers  &&
-                        (off >= ctx->size || ISNEWLINE(off))  &&  container.ch != _T('>'))
+                        (off >= ctx->size || ISNEWLINE(off))  &&  container.ch != _T('>')
+                        && container.ch != _T('!'))
             {
                 /* Noop. List mark followed by a blank line cannot interrupt a paragraph. */
             } else if(pivot_line->type == MD_LINE_TEXT  &&  n_parents == ctx->n_containers  &&
@@ -6051,6 +6178,10 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
                 line->beg = off;
                 line->data = container.ch;
 
+                /* First line in a BOX is an info string */
+                if(container.ch == _T('!'))
+                    line->type = MD_LINE_BOX_INFO;
+
                 /* Some of the following whitespace actually still belongs to the mark. */
                 if(off >= ctx->size || ISNEWLINE(off)) {
                     container.contents_indent++;
@@ -6140,6 +6271,12 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
             }
         }
 
+        /* box info string */
+        if(line->type == MD_LINE_BOX_INFO) {
+            while(off < ctx->size  &&  !ISNEWLINE(off)) off++;
+            break;
+        }
+
         /* By default, we are normal text line. */
         line->type = MD_LINE_TEXT;
         if(pivot_line->type == MD_LINE_TEXT  &&  n_brothers + n_children == 0) {
@@ -6234,7 +6371,7 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
     /* If we belong to a list after seeing a blank line, the list is loose. */
     if(prev_line_has_list_loosening_effect  &&  line->type != MD_LINE_BLANK  &&  n_parents + n_brothers > 0) {
         MD_CONTAINER* c = &ctx->containers[n_parents + n_brothers - 1];
-        if(c->ch != _T('>')) {
+        if(c->ch != _T('>') && c->ch != _T('!')) {
             MD_BLOCK* block = (MD_BLOCK*) (((char*)ctx->block_bytes) + c->block_byte_off);
             block->flags |= MD_BLOCK_LOOSE_LIST;
         }
@@ -6260,7 +6397,7 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
     }
 
     if(n_children > 0)
-        MD_CHECK(md_enter_child_containers(ctx, n_children));
+        MD_CHECK(md_enter_child_containers(ctx, n_children, line));
 
 abort:
     return ret;
@@ -6326,6 +6463,13 @@ md_process_line(MD_CTX* ctx, const MD_LINE_ANALYSIS** p_pivot_line, MD_LINE_ANAL
     if(line->type != pivot_line->type)
         MD_CHECK(md_end_current_block(ctx));
 
+    /* reset things and eat the MD_LINE_BOX_INFO line, aka the info strong for
+     * the box block */
+    if(line->type == MD_LINE_BOX_INFO) {
+        line->type = MD_LINE_BLANK;
+        return 0;
+    }
+
     /* The current line may start a new block. */
     if(ctx->current_block == NULL) {
         MD_CHECK(md_start_new_block(ctx, line));
diff --git a/src/md4c.h b/src/md4c.h
index 95f78f9..00265d9 100644
--- a/src/md4c.h
+++ b/src/md4c.h
@@ -58,6 +58,9 @@ typedef enum MD_BLOCKTYPE {
     /* <blockquote>...</blockquote> */
     MD_BLOCK_QUOTE,
 
+    /* <div class="box">...</div> */
+    MD_BLOCK_BOX,
+
     /* <ul>...</ul>
      * Detail: Structure MD_BLOCK_UL_DETAIL. */
     MD_BLOCK_UL,
@@ -280,6 +283,12 @@ typedef struct MD_BLOCK_TD_DETAIL {
     MD_ALIGN align;
 } MD_BLOCK_TD_DETAIL;
 
+/* Detailed info for MD_BLOCK_BOX. */
+typedef struct MB_BLOCK_BOX_DETAIL {
+    MD_ATTRIBUTE type;
+    MD_ATTRIBUTE title;
+} MD_BLOCK_BOX_DETAIL;
+
 /* Detailed info for MD_SPAN_A. */
 typedef struct MD_SPAN_A_DETAIL {
     MD_ATTRIBUTE href;
@@ -316,6 +325,7 @@ typedef struct MD_SPAN_WIKILINK {
 #define MD_FLAG_LATEXMATHSPANS              0x1000  /* Enable $ and $$ containing LaTeX equations. */
 #define MD_FLAG_WIKILINKS                   0x2000  /* Enable wiki links extension. */
 #define MD_FLAG_UNDERLINE                   0x4000  /* Enable underline extension (and disables '_' for normal emphasis). */
+#define MD_FLAG_BOX                         0x20000 /* Enable box extension */
 
 #define MD_FLAG_PERMISSIVEAUTOLINKS         (MD_FLAG_PERMISSIVEEMAILAUTOLINKS | MD_FLAG_PERMISSIVEURLAUTOLINKS | MD_FLAG_PERMISSIVEWWWAUTOLINKS)
 #define MD_FLAG_NOHTML                      (MD_FLAG_NOHTMLBLOCKS | MD_FLAG_NOHTMLSPANS)