更改:从:hostdiv[contenteditable=true]::before中删除:host

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<style>
            * {
                box-sizing: border-box;
                padding-bottom: 5px;
                margin-bottom: 10px;
            }

            :host #fBar {
                position: absolute;
                display: block;
            }

            :host div {
                position: relative;
                overflow: hidden;
                width: calc(100% - 10px);
                margin: 0 5px;
                height: 30px;
                background-color: white;
                /*border-bottom: solid 2px #1f1f1f;*/
                border-left: 1px solid #ddd;
                border-right: 1px solid #ddd;
                border-top: 1px solid #ddd;
                font-size: 15px;
                justify-content: center;
                font-weight: 200px;
                align-items: center;
                line-height: 25px;
                -webkit-box-shadow: 0px 1px 5px 0px rgba(0, 0, 0, 0.75);
                -moz-box-shadow: 0px 1px 5px 0px rgba(0, 0, 0, 0.75);
                box-shadow: 0px 1px 5px 0px rgba(0, 0, 0, 0.75);
            }


            div[contenteditable=true]::before {
                position: absolute;
                line-height: 25px;
                right: 5px;
                content: attr(placeholder);
                pointer-events: none;
                opacity: 0.6;
                color:blue;
                border:1px solid red;
            }
        </style>
        <div contenteditable="true" placeholder="I AM A PLACE HOLDER"></div>