(In reply to comment #5) 
> This is how Evolution is crashing as well (at least, it's the report for
> which we received a description and full backtrace, see the See Also field).

Sigh, I realize this is a private bug... I think thread 1 is probably the only important part; note the string "aria-" in the crash frame.

Core was generated by `/usr/bin/evolution'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  WTF::StringImpl::startsWith (this=0xbad801d800000002, matchString=matchString@entry=0x7f85ba36fea7 "aria-", matchLength=matchLength@entry=5, caseSensitive=caseSensitive@entry=true) at Source/WTF/wtf/text/StringImpl.cpp:1363
1363	    if (matchLength > length())
[Current thread is 1 (Thread 0x7f85c0247ac0 (LWP 17496))]

Thread 1 (Thread 0x7f85c0247ac0 (LWP 17496)):
#0  WTF::StringImpl::startsWith (this=0xbad801d800000002, matchString=matchString@entry=0x7f85ba36fea7 "aria-", matchLength=matchLength@entry=5, caseSensitive=caseSensitive@entry=true) at Source/WTF/wtf/text/StringImpl.cpp:1363
No locals.
#1  0x00007f85b8f3e00f in WTF::StringImpl::startsWith<6u> (caseSensitive=true, prefix=..., this=<optimized out>) at Source/WTF/wtf/text/StringImpl.h:730
No locals.
#2  WTF::String::startsWith<6u> (caseSensitive=true, prefix=..., this=<optimized out>) at Source/WTF/wtf/text/WTFString.h:281
No locals.
#3  WebCore::AXObjectCache::handleAttributeChanged (this=0x7f851b997f00, attrName=..., element=0x558fcfb67cb0) at Source/WebCore/accessibility/AXObjectCache.cpp:880
No locals.
#4  0x00007f85b91641ea in WebCore::Element::attributeChanged (this=0x558fcfb67cb0, name=..., oldValue=..., newValue=...) at Source/WebCore/dom/Element.cpp:1137
        cache = <optimized out>
        styleResolver = <optimized out>
        testShouldInvalidateStyle = true
        shouldInvalidateStyle = <optimized out>
#5  0x00007f85b9163520 in WebCore::Element::didModifyAttribute (this=this@entry=0x558fcfb67cb0, name=..., oldValue=..., newValue=...) at Source/WebCore/dom/Element.cpp:2851
No locals.
#6  0x00007f85b916b449 in WebCore::Element::setAttributeInternal (this=0x558fcfb67cb0, index=<optimized out>, name=..., newValue=..., inSynchronizationOfLazyAttribute=WebCore::Element::NotInSynchronizationOfLazyAttribute) at Source/WebCore/dom/Element.cpp:1075
        oldValue = {m_string = {m_impl = {m_ptr = 0x7f858c676000}}}
        valueChanged = <optimized out>
        attributeName = <optimized out>
#7  0x00007f85b91de4b9 in WebCore::CompositeEditCommand::applyCommandToComposite (this=this@entry=0x7f853a37c900, prpCommand=...) at Source/WebCore/editing/CompositeEditCommand.cpp:278
        command = {m_ptr = 0x7f853a56ad20}
#8  0x00007f85b91e4f1a in WebCore::CompositeEditCommand::setNodeAttribute (this=this@entry=0x7f853a37c900, element=..., attribute=..., value=...) at Source/WebCore/editing/CompositeEditCommand.cpp:664
No locals.
#9  0x00007f85b926c8f9 in WebCore::ReplaceSelectionCommand::removeRedundantStylesAndKeepStyleSpanInline (this=this@entry=0x7f853a37c900, insertedNodes=...) at Source/WebCore/editing/ReplaceSelectionCommand.cpp:525
        element = 0x558fcfb67cb0
        inlineStyle = 0x7f853a3cb410
        newInlineStyle = {m_ptr = 0x7f851b975b70}
        node = {m_ptr = 0x558fcfb67cb0}
        next = {m_ptr = 0x558fd14d9630}
#10 0x00007f85b926f714 in WebCore::ReplaceSelectionCommand::doApply (this=0x7f853a37c900) at Source/WebCore/editing/ReplaceSelectionCommand.cpp:1151
        selection = {m_base = {m_anchorNode = {m_ptr = 0x558fd16cd1b0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}, m_extent = {m_anchorNode = {m_ptr = 0x558fd16cd1b0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}, m_start = {m_anchorNode = {m_ptr = 0x558fd16cd1b0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}, m_end = {m_anchorNode = {m_ptr = 0x558fd16cd1b0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}, m_affinity = WebCore::DOWNSTREAM, m_selectionType = WebCore::VisibleSelection::CaretSelection, m_baseIsFirst = true, m_isDirectional = true}
        fragment = {m_document = {m_ptr = 0x7f853a276d00}, m_fragment = {m_ptr = 0x558fd0f6ad00}, m_hasInterchangeNewlineAtStart = false, m_hasInterchangeNewlineAtEnd = false}
        visibleStart = {m_deepPosition = {m_anchorNode = {m_ptr = 0x558fd16cd1b0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}, m_affinity = WebCore::DOWNSTREAM}
        visibleEnd = {m_deepPosition = {m_anchorNode = {m_ptr = 0x558fd16cd1b0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}, m_affinity = WebCore::DOWNSTREAM}
        selectionEndWasEndOfParagraph = true
        selectionStartWasStartOfParagraph = true
        startBlock = <optimized out>
        insertionPos = {m_anchorNode = {m_ptr = 0x558fd16cd1b0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}
        startIsInsideMailBlockquote = false
        selectionIsPlainText = <optimized out>
        currentRoot = <optimized out>
        endBR = <optimized out>
        originalVisPosBeforeEndBR = {m_deepPosition = {m_anchorNode = {m_ptr = 0x558fd02d50a0}, m_offset = 0, m_anchorType = 1, m_isLegacyEditingPosition = true}, m_affinity = WebCore::DOWNSTREAM}
        insertionBlock = {m_ptr = 0x558fcff56940}
        insertedNodes = {m_firstNodeInserted = {m_ptr = 0x558fcfb67cb0}, m_lastNodeInserted = {m_ptr = 0x558fd1a7b1f0}}
        refNode = {m_ptr = 0x558fd1a7b1f0}
        node = <optimized out>
        blockStart = <optimized out>
        plainTextFragment = false
        startOfInsertedContent = {m_deepPosition = {m_anchorNode = {m_ptr = 0x558fd14d9630}, m_offset = 0, m_anchorType = 0, m_isLegacyEditingPosition = true}, m_affinity = WebCore::DOWNSTREAM}
        lastPositionToSelect = {m_anchorNode = {m_ptr = 0x558fd1a7b1f0}, m_offset = -1204594578, m_anchorType = 5, m_isLegacyEditingPosition = false}
#11 0x00007f85b91de107 in WebCore::CompositeEditCommand::apply (this=0x7f853a37c900) at Source/WebCore/editing/CompositeEditCommand.cpp:227
No locals.
#12 0x00007f85b91de21d in WebCore::applyCommand (command=...) at Source/WebCore/editing/CompositeEditCommand.cpp:182
No locals.
#13 0x00007f85b9207946 in WebCore::executeInsertFragment (frame=..., fragment=...) at Source/WebCore/editing/EditorCommand.cpp:195
No locals.
#14 0x00007f85b9207df1 in WebCore::executeInsertHTML (frame=..., value=...) at Source/WebCore/editing/EditorCommand.cpp:508
No locals.
#15 0x00007f85b913c9df in WebCore::Document::execCommand (this=this@entry=0x7f853a276d00, commandName=..., userInterface=<optimized out>, value=...) at Source/WebCore/dom/Document.cpp:4227
No locals.
#16 0x00007f85b9e5d5dd in webkit_dom_document_exec_command (self=<optimized out>, command=<optimized out>, userInterface=userInterface@entry=0, value=0x558fd1496580 "<meta http-equiv=\"content-type\" content=\"text/html; charset=utf-8\"><span style=\"color: rgb(0, 0, 0); font-family: myriadpro-regular, Arial, sans-serif; font-size: 15px; font-style: normal; font-varian"...) at DerivedSources/webkitdom/WebKitDOMDocument.cpp:1157
        state = {m_previousState = 0x0}
        __PRETTY_FUNCTION__ = "gboolean webkit_dom_document_exec_command(WebKitDOMDocument*, const gchar*, gboolean, const gchar*)"
        item = 0x7f853a276d00
        convertedCommand = {m_impl = {m_ptr = 0x7f853a45fa50}}
        convertedValue = {m_impl = {m_ptr = 0x7f853a278700}}
        result = <optimized out>
#17 0x00007f85bfb7471b in e_html_editor_view_exec_command (view=view@entry=0x558fd0302790, command=command@entry=E_HTML_EDITOR_VIEW_COMMAND_INSERT_HTML, value=value@entry=0x558fd1496580 "<meta http-equiv=\"content-type\" content=\"text/html; charset=utf-8\"><span style=\"color: rgb(0, 0, 0); font-family: myriadpro-regular, Arial, sans-serif; font-size: 15px; font-style: normal; font-varian"...) at e-html-editor-view.c:8915
        document = <optimized out>
        cmd_str = <optimized out>
        has_value = <optimized out>
        __func__ = "e_html_editor_view_exec_command"
#18 0x00007f85bfb638ab in e_html_editor_selection_insert_html (selection=0x558fd15c6c90, html_text=0x558fd1496580 "<meta http-equiv=\"content-type\" content=\"text/html; charset=utf-8\"><span style=\"color: rgb(0, 0, 0); font-family: myriadpro-regular, Arial, sans-serif; font-size: 15px; font-style: normal; font-varian"...) at e-html-editor-selection.c:5673
        view = 0x558fd0302790
        command = E_HTML_EDITOR_VIEW_COMMAND_INSERT_HTML
        ev = 0x558fd13c0e00
        html_mode = -1
        __func__ = "e_html_editor_selection_insert_html"
#19 0x00007f859c2caca4 in e_composer_paste_html (r=<optimized out>, clipboard=<optimized out>) at e-composer-private.c:533
        editor = <optimized out>
        view = 0x558fd0302790
        editor_selection = 0x558fd15c6c90
        html = 0x558fd1496580 "<meta http-equiv=\"content-type\" content=\"text/html; charset=utf-8\"><span style=\"color: rgb(0, 0, 0); font-family: myriadpro-regular, Arial, sans-serif; font-size: 15px; font-style: normal; font-varian"...
        __func__ = "e_composer_paste_html"
#20 0x00007f85b7548d61 in request_targets_received_func (clipboard=0x558fcf7f5ee0, selection_data=selection_data@entry=0x7ffeb58e68a0, data=data@entry=0x558fd14b6fc0) at gtkclipboard.c:1325
        info = 0x558fd14b6fc0
        targets = 0x558fd0856fd0
        n_targets = 8
#21 0x00007f85b7548ff9 in selection_received (widget=0x558fcf5d1220, selection_data=0x7ffeb58e68a0, time=<optimized out>) at gtkclipboard.c:960
        request_info = 0x558fd1024e20
#22 0x00007f85b5b6c7a5 in g_closure_invoke (closure=0x558fcf1e9be0, return_value=return_value@entry=0x0, n_param_values=3, param_values=param_values@entry=0x7ffeb58e6530, invocation_hint=invocation_hint@entry=0x7ffeb58e64b0) at gclosure.c:801
        marshal = <optimized out>
        marshal_data = <optimized out>
        in_marshal = 0
        real_closure = 0x558fcf1e9bc0
        __func__ = "g_closure_invoke"
#23 0x00007f85b5b7e851 in signal_emit_unlocked_R (node=node@entry=0x558fccc28bc0, detail=detail@entry=0, instance=instance@entry=0x558fcf5d1220, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7ffeb58e6530) at gsignal.c:3627
        tmp = <optimized out>
        handler = 0x558fcf1e8f00
        accumulator = 0x0
        emission = {next = 0x7ffeb58e6ad0, instance = 0x558fcf5d1220, ihint = {signal_id = 70, detail = 0, run_type = G_SIGNAL_RUN_FIRST}, state = EMISSION_RUN, chain_type = 4}
        class_closure = 0x558fccc28b30
        handler_list = 0x558fcf1e8f00
        return_accu = 0x0
        accu = {g_type = 0, data = {{v_int = 0, v_uint = 0, v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, v_pointer = 0x0}, {v_int = 0, v_uint = 0, v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, v_pointer = 0x0}}}
        signal_id = 70
        max_sequential_handler_number = 110677
        return_value_altered = 0
#24 0x00007f85b5b87530 in g_signal_emit_valist (instance=instance@entry=0x558fcf5d1220, signal_id=signal_id@entry=70, detail=detail@entry=0, var_args=var_args@entry=0x7ffeb58e6768) at gsignal.c:3383
        instance_and_params = 0x7ffeb58e6530
        signal_return_type = <optimized out>
        param_values = 0x7ffeb58e6548
        node = <optimized out>
        i = <optimized out>
        n_params = <optimized out>
        __func__ = "g_signal_emit_valist"
#25 0x00007f85b5b87dc5 in g_signal_emit_by_name (instance=0x558fcf5d1220, detailed_signal=detailed_signal@entry=0x7f85b75c3e83 "selection-received") at gsignal.c:3479
        var_args = {{gp_offset = 32, fp_offset = 48, overflow_arg_area = 0x7ffeb58e68a0, reg_save_area = 0x7ffeb58e67b0}}
        detail = 0
        signal_id = 70
        itype = 94076144637376
        __func__ = "g_signal_emit_by_name"
#26 0x00007f85b746dd13 in gtk_selection_retrieval_report (info=info@entry=0x558fd1a7aef0, type=<optimized out>, format=<optimized out>, buffer=<optimized out>, length=length@entry=64, time=30218102) at gtkselection.c:3033
        data = {selection = 0x45, target = 0x95, type = 0x4, format = 32, data = 0x558fd14d96b0 "\225", length = 64, display = 0x558fccbf6020}
#27 0x00007f85b7471cbd in _gtk_selection_notify (widget=<optimized out>, event=0x558fd12b72f0) at gtkselection.c:2837
        tmp_list = 0x7f8534ecbd20
        info = 0x558fd1a7aef0
        window = <optimized out>
        buffer = 0x558fd14d96b0 "\225"
        length = 64
        type = 0x4
        format = 32
#28 0x00007f85b73e5fda in _gtk_marshal_BOOLEAN__BOXEDv (closure=0x558fccc28790, return_value=0x7ffeb58e6b00, instance=<optimized out>, args=<optimized out>, marshal_data=<optimized out>, n_params=<optimized out>, param_types=0x558fccc287c0) at gtkmarshalers.c:131
        cc = <optimized out>
        data1 = <optimized out>
        data2 = <optimized out>
        callback = <optimized out>
        arg0 = 0x558fd12b72f0
        args_copy = {{gp_offset = 32, fp_offset = 48, overflow_arg_area = 0x7ffeb58e6cb0, reg_save_area = 0x7ffeb58e6bf0}}
        v_return = <optimized out>
        __func__ = "_gtk_marshal_BOOLEAN__BOXEDv"
#29 0x00007f85b5b6c9d4 in _g_closure_invoke_va (closure=closure@entry=0x558fccc28790, return_value=return_value@entry=0x7ffeb58e6b00, instance=instance@entry=0x558fcf5d1220, args=args@entry=0x7ffeb58e6bd0, n_params=<optimized out>, param_types=0x558fccc287c0) at gclosure.c:864
        marshal = <optimized out>
        marshal_data = <optimized out>
        in_marshal = 0
        real_closure = 0x558fccc28770
        __func__ = "_g_closure_invoke_va"
#30 0x00007f85b5b86dd3 in g_signal_emit_valist (instance=0x558fcf5d1220, signal_id=<optimized out>, detail=0, var_args=var_args@entry=0x7ffeb58e6bd0) at gsignal.c:3292
        return_accu = 0x7ffeb58e6b00
        accu = {g_type = 20, data = {{v_int = 0, v_uint = 0, v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, v_pointer = 0x0}, {v_int = 0, v_uint = 0, v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, v_pointer = 0x0}}}
        accumulator = 0x558fccc287e0
        emission = {next = 0x0, instance = 0x558fcf5d1220, ihint = {signal_id = 69, detail = 0, run_type = G_SIGNAL_RUN_LAST}, state = EMISSION_RUN, chain_type = 94076144637376}
        signal_id = 69
        instance_type = 94076144637376
        emission_return = {g_type = 20, data = {{v_int = 0, v_uint = 0, v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, v_pointer = 0x0}, {v_int = 0, v_uint = 0, v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, v_pointer = 0x0}}}
        rtype = 20
        static_scope = 0
        fastpath_handler = <optimized out>
        closure = <optimized out>
        run_type = <optimized out>
        l = <optimized out>
        fastpath = <optimized out>
        instance_and_params = <optimized out>
        signal_return_type = <optimized out>
        param_values = <optimized out>
        node = <optimized out>
        i = <optimized out>
        n_params = <optimized out>
        __func__ = "g_signal_emit_valist"
#31 0x00007f85b5b878ff in g_signal_emit (instance=instance@entry=0x558fcf5d1220, signal_id=<optimized out>, detail=detail@entry=0) at gsignal.c:3439
        var_args = {{gp_offset = 24, fp_offset = 48, overflow_arg_area = 0x7ffeb58e6cb0, reg_save_area = 0x7ffeb58e6bf0}}
#32 0x00007f85b75234bc in gtk_widget_event_internal (widget=0x558fcf5d1220, event=0x558fd12b72f0) at gtkwidget.c:7692
        signal_num = <optimized out>
        return_val = 0
        handled = 0
        event = 0x558fd12b72f0
        widget = 0x558fcf5d1220
#33 0x00007f85b73e50b6 in gtk_main_do_event (event=0x558fd12b72f0) at gtkmain.c:1795
        event_widget = 0x558fcf5d1220
        grab_widget = 0x558fcf5d1220
        topmost_widget = <optimized out>
        window_group = 0x558fd0131840
        rewritten_event = <optimized out>
        device = 0x0
        tmp_list = <optimized out>
        __func__ = "gtk_main_do_event"
#34 0x00007f85b6f5de92 in gdk_event_source_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at gdkeventsource.c:369
        display = <optimized out>
        event = <optimized out>
#35 0x00007f85b586de3a in g_main_dispatch (context=0x558fccbe7840) at gmain.c:3154
        dispatch = 0x7f85b6f5de70 <gdk_event_source_dispatch>
        prev_source = 0x0
        was_in_call = 0
        user_data = 0x0
        callback = 0x0
        cb_funcs = 0x0
        cb_data = 0x0
        need_destroy = <optimized out>
        source = 0x558fccbe7750
        current = 0x558fccc54d40
        i = 0
#36 g_main_context_dispatch (context=context@entry=0x558fccbe7840) at gmain.c:3769
No locals.
#37 0x00007f85b586e1d0 in g_main_context_iterate (context=0x558fccbe7840, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3840
        max_priority = 0
        timeout = 0
        some_ready = 1
        nfds = <optimized out>
        allocated_nfds = 4
        fds = 0x558fcf754da0
#38 0x00007f85b586e4f2 in g_main_loop_run (loop=0x558fcf32d5d0) at gmain.c:4034
        __func__ = "g_main_loop_run"
#39 0x00007f85b73e4325 in gtk_main () at gtkmain.c:1241
        loop = 0x558fcf32d5d0
#40 0x0000558fcc45d089 in main (argc=1, argv=0x7ffeb58e7008) at main.c:660
        shell = 0x558fcccc11e0
        settings = <optimized out>
        error = 0x0


We also have a valgrind log (available on request); I think the main interesting part is:

==21341== Conditional jump or move depends on uninitialised value(s)
==21341==    at 0xCEBA8FF: WTF::fastFree(void*) (in /usr/lib64/libjavascriptcoregtk-3.0.so.0.16.18)
==21341==    by 0xB15E1A4: ??? (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0xB15DA6B: ??? (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0xB15F451: ??? (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0xA9F7B32: ??? (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0xA9F8173: ??? (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0xA9FA28D: ??? (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0xA9FA9D1: ??? (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0xB71B5DC: webkit_dom_document_exec_command (in /usr/lib64/libwebkitgtk-3.0.so.0.22.16)
==21341==    by 0x5173D20: ??? (in /usr/lib64/evolution/libevolution-util.so)
==21341==    by 0xF1D4BEA: g_type_create_instance (in /usr/lib64/libgobject-2.0.so.0.4600.2)
==21341==    by 0xF1B6B7A: ??? (in /usr/lib64/libgobject-2.0.so.0.4600.2)